Thyroid hormone as a determinant of metabolic and contractile phenotype of skeletal muscle

Exploring the potential regulation of DUOX in thyroid hormone‑autophagy signaling via IGF‑1 in the skeletal muscle (Review)

Dual oxidases (DUOX) are enzymes that have the main function in producing reactive oxygen species (ROS) in various tissues. DUOX also play an important role in the synthesis of H2O2, which is essential for the production of thyroid hormone. Thyroid hormones can influence the process of muscle development through direct stimulation of ROS, 5' AMP-activated protein kinase (AMPK) and mTOR and indirect effect autophagy and the insulin-like growth factor 1 (IGF-1) pathway. IGF-1 signaling controls autophagy in two ways: Inhibiting autophagy through activation of the PI3K/AKT/mTOR/MAPK pathway and promoting mitophagy through the nuclear factor erythroid 2-related factor 2-binding receptor Bcl2/adenovirus E1B 19 kDa protein-interacting protein 3. Thyroid hormone deficiency caused by the absence of DUOX should be considered because it might have a significant effect on the growth of skeletal muscle. The effect of DUOX regulation on thyroid hormone autophagy via IGF-1 in skeletal muscle has not been well investigated. The present review discussed the regulatory interactions between DUOX, thyroid hormone, IGF-1 and autophagy, which can influence skeletal muscle development.

Methimazole disrupted skeletal ossification and muscle fiber transition in Bufo gargarizans larvae

Methimazole (MMI) is an emerging endocrine disrupting chemical (EDC) due to its increasing use in the treatment of thyrotoxicosis (hyperthyroidism), but its potential impact on amphibian development remains largely unexplored. In the present study, the effects of 8 mg/L MMI and 1 μg/L thyroxine (T4) exposure on skeletal ossification and muscle development in Bufo gargarizans tadpoles were comprehensively investigated by double skeletal staining, histological analysis and RNA sequencing. Our results indicated that MMI treatment down-regulated the expression levels of ossification-related genes (e.g., BMPs, MMPs, and Wnt9a) in cartilage, thereby delaying chondrocyte apoptosis and inhibiting hindlimb ossification. Muscle sarcomere was elongated in both the MMI and T4 treatment groups, which may lead to muscle weakness and consequently affect land motion. Additionally, we evaluated the expression levels of fast muscle-related genes (TNNI2 and TNNT3) and slow muscle-related genes (TNNI1 and TNNT1), revealing an opposite trend in the transition from fast to slow muscle after T4 and MMI exposures. In conclusion, these findings fill the data gap regarding MMI contamination in aquatic environments by revealing the negative effects of MMI on amphibian bone and muscle development. Future studies should address the toxicity of EDCs to wildlife and inform aquatic ecosystem conservation strategies.

Exploring the association between muscle mass and thyroid function in Chinese community subjects over 45 years old with normal thyroid function: a cross-sectional analysis

Background

Currently, nothing is known about the connection between muscle mass and thyroid hormone levels in middle-aged and elderly Chinese with normal thyroid function. The purpose of this study was to determine the potential association between muscle mass and thyroid function status in middle-aged and elderly Chinese subjects with normal thyroid function.

Methods

A cohort of 1868 participants in China were included in this retrospective study; their mean age was 53.97 years, and their skeletal muscle mass index was 7.44 kg/m2. Of them, 60.97% were men. Thyroid hormone concentrations, standard biochemical indices, and the frequency of chronic illnesses were among the many factors that were evaluated. Bioelectrical impedance analysis (BIA) was used to assess the patients' body composition. The skeletal muscle index (SMI) was calculated using the following formula: SMI = ASM (kg)/height 2 (m2), where ASM stands for appendicular skeletal muscle mass. To identify the correlations between the variables, the Spearman correlation coefficient was used. Binary logistic regression analysis was conducted to investigate the potential linkages between thyroid hormone levels and diminished muscle mass.

Results

In this investigation, a significant correlation was observed between low muscle mass and FT3/FT4 (OR=0.044, 95% CI: 0.004-0.440, P=0.008), as well as FT3 (OR=0.697, 95% CI: 0.508-0.957, P=0.025). Conversely, no discernible correlation trend was detected with TSH (OR=0.972, 95% CI: 0.814-1.160, P=0.753) and FT4 (OR=1.97, 95% CI=0.983-1.224, P=0.1). Following adjustment for various confounding factors, including age, vitamin D levels, triglycerides, HDL-C, LDL-C, total protein, hypertension, diabetes, hyperuricemia, and overweight/obesity, across the entire study population, a positive correlation between SMI and FT3/FT4 was identified. Subsequent gender, age, and weight-stratified analyses revealed consistent correlation trends between SMI and FT3/FT4, with all interactions yielding P-values > 0.05.

Conclusion

Our study has revealed that among middle-aged and elderly Chinese individuals exhibiting normal thyroid function, a reduction in the free T3 to free T4 ratio is associated with a decline in muscle mass.

Effects of thyroid hormone on monochromatic light combinations mediate skeletal muscle fiber pattern in broilers

It has been shown that monochromatic green light and blue light promote skeletal muscle development in early (P0-P26) and later growth stages (P27-P42), respectively. This study further investigated the effects of monochromatic light combinations on myogenesis and myofiber types transformation in broilers. Here, a total of 252 chicks were exposed to monochromatic light [red (R), green (G), blue (B), or white light (W)], and monochromatic light combination [green and blue light combination (GB), blue and green light combination (BG), red and blue combination (RB)] until P42. Compared with other groups, GB significantly increased body weight, and muscle organ index, both proportions of larger-size myofibers and oxidative myofibers in the pectoralis major (PM) and gastrocnemius muscle (GAS). Meanwhile, GB up-regulated the abundance of oxidative genes MYH7B and MYH1B, transcription factors PAX7 and Myf5, antioxidant proteins Nrf2, HO-1, and GPX4, and the activities of antioxidant enzymes CAT, GPx, and T-AOC, but down-regulated the abundance of glycolytic related genes MYH 1A, MyoD, MyoG, Mstn, Keap1, TNFa, and MDA levels. Consistent with the change of myofiber pattern, GB significantly reduced serum thyroid hormone (TH) levels, up-regulated skeletal muscle deiodinase DIO3 expression and down-regulated deiodinase DIO2 expression, which may directly lead to the reduction of intramuscular TH levels to affect myofiber types transformation. In contrast, the proportion of fast glycolytic muscle fibers increased in the RR with increasing TH levels. After thyroidectomy, the above parameters were inversed and resulted in no significant difference of each color light treatment group. These data suggested that GB significantly increased the proportion of oxidative muscle fibers and antioxidant capacity in skeletal muscle of broilers, which was regulated by TH-DIO2/DIO3 signaling pathway.

Associations between short-term exposure to air pollution and thyroid function in a representative sample of the Korean population

BACKGROUND

Disruption of thyroid function can profoundly affect various organ systems. However, studies on the association between air pollution and thyroid function are relatively scarce and most studies have focused on the long-term effects of air pollution among pregnant women.

OBJECTIVES

This study aimed to explore the associations between short-term exposure to air pollution and thyroid function in the general population.

METHODS

Data from the Korea National Health and Nutrition Examination Survey (2013-2015) were analyzed (n = 5,626). Air pollution concentrations in residential addresses were estimated using Community Multiscale Air Quality models. The moving averages of air pollution over 7 days were set as exposure variables through exploratory analyses. Linear regression and quantile g-computation models were constructed to assess the effects of individual air pollutants and air pollution mixture, respectively.

RESULTS

A 10-ppb increase in NO2 (18.8-μg/m3 increase) and CO (11.5-μg/m3 increase) was associated with 2.43% [95% confidence interval (CI): 0.42, 4.48] and 0.19% (95% CI: 0.01, 0.36) higher thyroid-stimulating hormone (TSH) levels, respectively. A 10-μg/m3 increase in PM2.5 and a 10-ppb increase in O3 (19.6-μg/m3 increment) were associated with 0.87% (95% CI: 1.47, -0.27) and 0.59% (95% CI: 1.18, -0.001) lower free thyroxine (fT4) levels, respectively. A simultaneous quartile increase in PM2.5, NO2, O3, and CO levels was associated with lower fT4 but not TSH levels.

CONCLUSIONS

As the subtle changes in thyroid function can affect various organ systems, the present results may have substantial public health implications despite the relatively modest effect sizes. Because this was a cross-sectional study, it is necessary to conduct further experimental or repeated-measures studies to consolidate the current results.

The key roles of thyroid hormone in mitochondrial regulation, at interface of human health and disease

Mitochondria are highly plastic and dynamic organelles long known as the powerhouse of cellular bioenergetics, but also endowed with a critical role in stress responses and homeostasis maintenance, supporting and integrating activities across multifaced cellular processes. As a such, mitochondria dysfunctions are leading causes of a wide range of diseases and pathologies. Thyroid hormones (THs) are endocrine regulators of cellular metabolism, regulating intracellular nutrients fueling of sugars, amino acids and fatty acids. For instance, THs regulate the balance between the anabolism and catabolism of all the macro-molecules, influencing energy homeostasis during different nutritional conditions. Noteworthy, not only most of the TH-dependent metabolic modulations act via the mitochondria, but also THs have been proved to regulate the mitochondrial biosynthesis, dynamics and function. The significance of such an interplay is different in the context of specific tissues and strongly impacts on cellular homeostasis. Thus, a comprehensive understanding of THs-dependent mitochondrial functions and dynamics is required to develop more precise strategies for targeting mitochondrial function. Herein, we describe the mechanisms of TH-dependent metabolic regulation with a focus on mitochondrial action, in different tissue contexts, thus providing new insights for targeted modulation of mitochondrial dynamics.

Effects of thyroid hormones in skeletal muscle protein turnover

Thyroid hormones (THs) are critical regulators of muscle metabolism in both healthy and unhealthy conditions. Acting concurrently as powerful anabolic and catabolic factors, THs are endowed with a vital role in muscle mass maintenance. As a result, thyroid dysfunctions are the leading cause of a wide range of muscle pathologies, globally identified as myopathies. Whether muscle wasting is a common feature in patients with hyperthyroidism and is mainly caused by THs-dependent stimulation of muscle proteolysis, also muscle growth is often associated with hyperthyroid conditions, linked to THs-dependent stimulation of muscle protein synthesis. Noteworthy, also hypothyroid status negatively impacts on muscle physiology, causing muscle weakness and fatigue. Most of these symptoms are due to altered balance between muscle protein synthesis and breakdown. Thus, a comprehensive understanding of THs-dependent skeletal muscle protein turnover might facilitate the management of physical discomfort or weakness in conditions of thyroid disease. Herein, we describe the molecular mechanisms underlying the THs-dependent alteration of skeletal muscle structure and function associated with muscle atrophy and hypertrophy, thus providing new insights for targeted modulation of skeletal muscle dynamics.

Dietary protein restriction regulates skeletal muscle fiber metabolic characteristics associated with the FGF21-ERK1/2 pathway

Under conditions of dietary amino acid balance, decreasing the dietary crude protein (CP) level in pigs has a beneficial effect on meat quality. To further elucidate the mechanism, we explored the alteration of muscle fiber characteristics and key regulators related to myogenesis in the skeletal muscle of pigs fed a protein restricted diet. Compared to pigs fed a normal protein diet, dietary protein restriction significantly increased the slow-twitch muscle fiber proportion in skeletal muscle, succinic dehydrogenase (SDH) activity, the concentrations of ascorbate, biotin, palmitoleic acid, and the ratio of s-adenosylhomocysteine (SAM) to s-adenosylhomocysteine (SAH), but the fast-twitch muscle fiber proportion, lactate dehydrogenase (LDH) activity, the concentrations of ATP, glucose-6-phosphate, SAM, and SAH in skeletal muscle, and the ratio of serum triiodothyronine (T3) to tetraiodothyronine (T4) were decreased. In conclusion, we demonstrated that dietary protein restriction induced skeletal muscle fiber remodeling association the regulation of FGF21-ERK1/2-mTORC1 signaling in weaned piglets.

Serum FT3/FT4, but not TSH is associated with handgrip strength in euthyroid U.S. population: evidence from NHANES

Objective

Although several studies have examined the relationship between thyroid function and muscle strength, their population primarily derived from Asian areas, and their results were controversial. Thus, this study aimed to explore the association between thyroid function and handgrip strength (HGS) in the U.S. population.

Methods

A total of 1,067 participants from NHANES were categorized into three different age groups including young (<45 years), middle (45~64 years), and old (≥65 years) age groups. Thyroid function was measured by the competitive binding immune-enzymatic assays, while HGS was examined by a trained evaluator using a dynamometer. The weighted multiple linear regression models were used to examine the association between thyroid function and handgrip strength. The restricted cubic splines were employed to explore the non-linear relationship between these two variables. All statistical analyses were performed using the SPSS version 20.0 and R software.

Results

After adjustment for potential covariates, FT3/FT4, but not TSH was positively associated with HGS in middle age group (β=0.091, t=2.428, P=0.016). The subgroup analysis by sex revealed that the positive association between FT3/FT4 and HGS was observed in the middle age group for both male and female participants (β=0.163, t=2.121, P=0.035; β=0.157, t=2.180, P=0.031). The RCS analysis showed a statistically significant non-linear association between FT3/FT4 and HGS in overall population (P for non-linear=0.026). After adjustment for covariates, men with low HGS had a significant lower FT3/FT4 than those without low HGS in old age group (P=0.013). There was a significant increase in TSH level for female participants with low HGS in old age group compared to those with normal HGS (P=0.048).

Conclusions

This study demonstrated FT3/FT4, but not TSH, was positively associated with HGS in middle age group, and the different association was observed in men in middle age group when participants were stratified by sex. Future longitudinal cohort study should be conducted to reveal the causal relationship between thyroid function and muscle strength.

Associations between thyroid hormones and appendicular skeletal muscle index, and hand grip strength in people with diabetes: The KAMOGAWA-A study

AIM

To assess the effects of thyroid hormones on appendicular skeletal muscle index (SMI) and hand grip strength (HGS) in people with diabetes.

METHODS

This cross-sectional cohort included 1,135 participants with diabetes admitted to 3 hospitals in Japan. Multiple regression analysis was performed to determine the associations among thyroid hormone levels, SMI, and HGS.

RESULTS

Of the 1,135 participants, 480 were female. Their median (interquartile range) age, body mass index, durations of diabetes, and glycated haemoglobin levels were 68 years, 24.3 kg/m2, 10 years, and 7.6 %, respectively. The median (interquartile range) SMI (kg/m2) and hand grip strength of the cohort were 7.1 kg/m2 and 28.2 kg, respectively. Positive correlations between FT3 and the FT3/FT4 ratio with SMI and HGS was observed after adjusting for covariates in males. A negative correlation was found between the FT3/FT4 ratio and sarcopenia as a result of low SMI and low HGS in the male participants but not in females (p for interaction = 0.02).

CONCLUSIONS

FT3/FT4 ratios may impact skeletal muscles in people with diabetes-particularly in males. Assessments of FT3/FT4 ratios may represent key indicators of muscle mass and strength in males.

Exploring the Association between Thyroid Function and Frailty: Insights from Representative Korean Data

BACKGRUOUND

This study investigates the association between thyroid function and frailty in the old patients using representative data.

METHODS

The study was conducted using data from the Korea National Health and Nutrition Examination Survey conducted from 2013 to 2015. The study population included 2,416 participants aged 50 years and older with available thyroid function test data. Frailty assessment was performed using the Fried frailty phenotype. The prevalence of frailty was analyzed across different thyroid diseases and thyroid function parameters.

RESULTS

The significant association between thyroid dysfunction and frailty was observed in overt hyperthyroidism and subclinical hyperthyroidism. After adjusting for various factors, the association between thyroid dysfunction and frailty remained significant. On the other hand, overt hypothyroidism did not show a significant association with frailty in the adjusted analysis. For individuals with overt hyperthyroidism and subclinical hyperthyroidism, higher levels of free thyroxine (FT4) were significantly associated with an increased risk of frailty (aOR >999; 95% CI, >999 to 999). Among individuals with overt hypothyroidism, lower level of FT4 levels and high thyrotropin (TSH) levels showed a significant association with frailty risk (FT4: aOR, <0.01; TSH: aOR, 999). In participants with subclinical hypothyroidism, there were no significant associations between parameters for thyroid and frailty risk.

CONCLUSION

These findings suggest that thyroid dysfunction, particularly overt hyperthyroidism and subclinical hyperthyroidism, may be associated with an increased risk of frailty in the old patients.

TT3, a More Practical Indicator for Evaluating the Relationship Between Sarcopenia and Thyroid Hormone in the Euthyroid Elderly Compared with FT3

BACKGROUND AND AIMS

Sarcopenia is a common disease in the elderly, and the thyroid hormone (TH) might participate in the pathogenesis of sarcopenia. However, the results of previous studies were not completely consistent. We performed this study to investigate the association between THs and sarcopenia in a Chinese elderly euthyroid population.

SUBJECTS AND METHODS

A total of 309 Chinese elderly euthyroid subjects with an average age of 85.19 ± 7.8 years were enrolled. Participants were divided into four groups (non-sarcopenia, possible sarcopenia, sarcopenia and serve sarcopenia) according to the consensus update of AWGS in 2019. Serum levels of TT3, FT3, TT4, FT4, TSH, rT3 and TBG were measured. Muscle mass was measured by multifrequency bioelectrical impedance analysis, hand grip (HG) was represented by spring-type dynamometer, and gait speed (GS) was determined by 6-metre walk test. The FRAIL scale was used to assess frailty.

RESULTS

Compared to the non-sarcopenia group, the sarcopenia group showed a significant increase in age and FRIAL score, while FT3 and TT3 levels decreased significantly. Partial correlation analysis (adjusted by age, gender and the scores of FRIAL scale) indicated that FT3, TT3 and TSH had significant positive correlations with HG, and there also was a significant positive correlation between TT3 and GS. In addition, after adjusting for age, gender, BMI, ALT, sCr, and score on the FRAIL scale, the multivariate linear regression analysis showed that TT3 was positively associated with muscle strength and negatively associated with sarcopenia risk.

CONCLUSION

There is an association between the low TT3 level and sarcopenia. Therefore, maintaining higher T3 concentrations within the normal range appears to be beneficial for sarcopenia in the elderly. In addition, due to the fluctuation of FT3, TT3 is a more stable and practical indicator to evaluate the relationship between sarcopenia and thyroid hormone in the elderly euthyroid population.

Thyroid Axis and Vestibular Physiopathology: From Animal Model to Pathology

A recent work of our group has shown the significant effects of thyroxine treatment on the restoration of postural balance function in a rodent model of acute peripheral vestibulopathy. Based on these findings, we attempt to shed light in this review on the interaction between the hypothalamic-pituitary-thyroid axis and the vestibular system in normal and pathological situations. Pubmed database and relevant websites were searched from inception through to 4 February 2023. All studies relevant to each subsection of this review have been included. After describing the role of thyroid hormones in the development of the inner ear, we investigated the possible link between the thyroid axis and the vestibular system in normal and pathological conditions. The mechanisms and cellular sites of action of thyroid hormones on animal models of vestibulopathy are postulated and therapeutic options are proposed. In view of their pleiotropic action, thyroid hormones represent a target of choice to promote vestibular compensation at different levels. However, very few studies have investigated the relationship between thyroid hormones and the vestibular system. It seems then important to more extensively investigate the link between the endocrine system and the vestibule in order to better understand the vestibular physiopathology and to find new therapeutic leads.

Can animals tune tissue mechanics in response to changing environments caused by anthropogenic impacts?

Anthropogenic climate change and pollution are impacting environments across the globe. This Review summarises the potential impact of such anthropogenic effects on animal tissue mechanics, given the consequences for animal locomotor performance and behaviour. More specifically, in light of current literature, this Review focuses on evaluating the acute and chronic effects of temperature on the mechanical function of muscle tissues. For ectotherms, maximal muscle performance typically occurs at temperatures approximating the natural environment of the species. However, species vary in their ability to acclimate to chronic changes in temperature, which is likely to have longer-term effects on species range. Some species undergo periods of dormancy to avoid extreme temperature or drought. Whilst the skeletal muscle of such species generally appears to be adapted to minimise muscle atrophy and maintain performance for emergence from dormancy, the increased occurrence of extreme climatic conditions may reduce the survival of individuals in such environments. This Review also considers the likely impact of anthropogenic pollutants, such as hormones and heavy metals, on animal tissue mechanics, noting the relative paucity of literature directly investigating this key area. Future work needs to determine the direct effects of anthropogenic environmental changes on animal tissues and related changes in locomotor performance and behaviour, including accounting for currently unknown interactions between environmental factors, e.g. temperature and pollutants.

Ferulic acid supplementation for 40 days in hair ewe lambs experiencing seasonal heat stress: short-term effects on physiological responses, growth, metabolism, and hematological profile

Free ferulic acid (FA) is a natural compound with antioxidant properties which mitigates the negative effects of cold stress in sheep; however, its impact on thermoregulatory responses in heat-stressed sheep has not been defined. The objective was to evaluate the effects of FA supplementation on physiological responses, serum analyte concentrations, and the hematological profile of heat-stressed hair ewe lambs. Twenty-two Dorper × Katahdin ewe lambs (initial body weight = 23.5 ± 2.8 kg and age = 4 months) were housed in individual pens for 40 days and assigned under a randomized complete block design to the following treatments (n = 11): basal diet with 0 (control) or 250 mg of FA/kg of feed. The FA × sampling day interaction only affected serum concentration of some metabolic hormones; particularly on day 20 of the trial, FA increased (P < 0.01) insulins and the insulin to glucose ratio while decreased (P = 0.05) thyroxine. Overall, supplemental FA did not affect rectal temperature, respiratory rate, most body surface temperatures, feedlot performance, and serum concentrations of metabolites, electrolytes, triiodothyronine, and cortisol. In addition, FA only tended to decrease (P ≥ 0.09) erythrocyte count and plaquetocrit and to increase (P = 0.08) mean corpuscular volume. In conclusion, FA supplementation did not improve the growth nor thermoregulatory capacity of heat-stressed hair ewe lambs. Still, it partially modulated the metabolism to reinforce some energetic adaptive mechanisms when the ambient temperature was ≥ 35 °C.

Regulatory Mechanisms of Muscle Mass: The Critical Role of Resistance Training in Children and Adolescent

Muscle mass and strength are subjected to several regulations. We found endocrine signals such as growth hormone, insulin-like growth factor 1, testosterone, thyroid hormones, and glucocorticoids among them. Neural inputs also influence muscle development, modulating mass and strength. Among the external stimuli that modulate these muscular features is physical training such as resistance and endurance training. Specifically, resistance training can mediate an increase in muscle mass by hypertrophy in adults, but the effects in children and adolescents are full of myths for most of the population. However, the evidence shows that the impact of resistance training on children and adolescents is clear and provides a wide range of benefits. However, qualified professionals must be available since exercise prescription and subsequent supervision must follow this population's abilities, needs, and interests.

Organotypic cultures as aging associated disease models

Aging remains a primary risk factor for a host of diseases, including leading causes of death. Aging and associated diseases are inherently multifactorial, with numerous contributing factors and phenotypes at the molecular, cellular, tissue, and organismal scales. Despite the complexity of aging phenomena, models currently used in aging research possess limitations. Frequently used in vivo models often have important physiological differences, age at different rates, or are genetically engineered to match late disease phenotypes rather than early causes. Conversely, routinely used in vitro models lack the complex tissue-scale and systemic cues that are disrupted in aging. To fill in gaps between in vivo and traditional in vitro models, researchers have increasingly been turning to organotypic models, which provide increased physiological relevance with the accessibility and control of in vitro context. While powerful tools, the development of these models is a field of its own, and many aging researchers may be unaware of recent progress in organotypic models, or hesitant to include these models in their own work. In this review, we describe recent progress in tissue engineering applied to organotypic models, highlighting examples explicitly linked to aging and associated disease, as well as examples of models that are relevant to aging. We specifically highlight progress made in skin, gut, and skeletal muscle, and describe how recently demonstrated models have been used for aging studies or similar phenotypes. Throughout, this review emphasizes the accessibility of these models and aims to provide a resource for researchers seeking to leverage these powerful tools.

Thyroid Hormone Abuse in Elite Sports: The Regulatory Challenge

Abuse of androgens and erythropoietin has led to hormones being the most effective and frequent class of ergogenic substances prohibited in elite sports by the World Anti-Doping Agency (WADA). At present, thyroid hormone (TH) abuse is not prohibited, but its prevalence among elite athletes and nonprohibited status remains controversial. A corollary of prohibiting hormones for elite sports is that endocrinologists must be aware of a professional athlete's risk of disqualification for using prohibited hormones and/or to certify Therapeutic Use Exemptions, which allow individual athletes to use prohibited substances for valid medical indications. This narrative review considers the status of TH within the framework of the WADA Code criteria for prohibiting substances, which requires meeting 2 of 3 equally important criteria of potential performance enhancement, harmfulness to health, and violation of the spirit of sport. In considering the valid clinical uses of TH, the prevalence of TH use among young adults, the reason why some athletes seek to use TH, and the pathophysiology of sought-after and adverse effects of TH abuse, together with the challenges of detecting TH abuse, it can be concluded that, on the basis of present data, prohibition of TH in elite sport is neither justified nor feasible.

Hypothyroidism Is Correlated with Ventilator Complications and Longer Hospital Days after Coronary Artery Bypass Grafting Surgery in a Relatively Young Population: A Nationwide, Population-Based Study

Background: Some research indicated that hypothyroidism has huge adverse effects for the metabolic, cardiovascular, respiratory, and immune systems. However, there is no confirmed conclusion for the effect of cardiovascular surgery. This cohort study aims to investigate the prognosis of hypothyroidism patient at the age under 65-year-old after coronary artery bypass grafting (CABG) surgery. Method: From the National Health Insurance Research Database of Taiwan, 1586 patients with hypothyroidism who underwent elective CABG surgery were selected, along with 6334 patients who underwent surgery in a ratio of 1:4 sex-, age- and index year-matched controls, who were out of hypothyroidism. We used Cox proportional hazard analysis to compare the rate of 30-day, 5-year mortality, post-operative atrial fibrillation, respiratory complication during an average of 10-year follow-up. Result: Post-CABG patients had more hospital days, which was associated with hypothyroidism, male, DM and higher CCI_R (p < 0.001). Post-CABG patients had more inpatient respiratory complications, which was associated with hypothyroidism (p = 0.041), DM and CCI_R (p < 0.001, p = 0.046), and there was no difference in 1-year respiratory complication, tracheostomy in the same hospital course and within 1 year, repeated PCI, Af, CVVH, cerebral infarction, 30-day and 5-year mortality rate. Conclusions: Hypothyroidism correlates to post-CABG ventilator-related complications and pneumonia, and prolonged hospital days, but no effect on 30-day, 5-year mortality, post-operative atrial fibrillation and cerebral infarction rate. Thyroid function survey might include routinely preoperative survey for CABG outcome prognosis.

Modulation of Deiodinase Types 2 and 3 during Skeletal Muscle Regeneration

The muscle stem-cell niche comprises numerous cell types, which coordinate the regeneration process after injury. Thyroid hormones are one of the main factors that regulate genes linked to skeletal muscle. In this way, deiodinase types 2 and 3 are responsible for the fine-tuning regulation of the local T3 amount. Although their expression and activity have already been identified during muscle regeneration, it is of utmost importance to identify the cell type and temporal pattern of expression after injury to thoroughly comprehend their therapeutic potential. Here, we confirmed the expression of Dio2 and Dio3 in the whole tibialis anterior muscle. We identified, on a single-cell basis, that Dio2 is present in paired box 7 (PAX7)-positive cells starting from day 5 after injury. Dio2 is present in platelet derived growth factor subunit A (PDGFA)-expressing fibro-adipogenic progenitor cells between days 7 and 14 after injury. Dio3 is detected in myogenic differentiation (MYOD)-positive stem cells and in macrophages immediately post injury and thereafter. Interestingly, Dio2 and Dio3 RNA do not appear to be present in the same type of cell throughout the process. These results provide further insight into previously unseen aspects of the crosstalk and synchronized regulation of T3 in injured muscle mediated by deiodinases. The set of findings described here further define the role of deiodinases in muscle repair, shedding light on potential new forms of treatment for sarcopenia and other muscular diseases.

Thyroid Hormone Receptor Isoforms Alpha and Beta Play Convergent Roles in Muscle Physiology and Metabolic Regulation

Skeletal muscle is a key energy-regulating organ, skilled in rapidly boosting the rate of energy production and substrate consumption following increased workload demand. The alteration of skeletal muscle metabolism is directly associated with numerous pathologies and disorders. Thyroid hormones (THs) and their receptors (TRs, namely, TRα and TRβ) exert pleiotropic functions in almost all cells and tissues. Skeletal muscle is a major THs-target tissue and alterations of THs levels have multiple influences on the latter. However, the biological role of THs and TRs in orchestrating metabolic pathways in skeletal muscle has only recently started to be addressed. The purpose of this paper is to investigate the muscle metabolic response to TRs abrogation, by using two different mouse models of global TRα- and TRβKO. In line with the clinical features of resistance to THs syndromes in humans, characterized by THRs gene mutations, both animal models of TRs deficiency exhibit developmental delay and mitochondrial dysfunctions. Moreover, using transcriptomic and metabolomic approaches, we found that the TRs–THs complex regulates the Fatty Acids (FAs)-binding protein GOT2, affecting FAs oxidation and transport in skeletal muscle. In conclusion, these results underline a new metabolic role of THs in governing muscle lipids distribution and metabolism.

Thyroid metabolism and supplementation. A review framed in sports environment

OBJECTIVES

This paper aimed to consider those features that may suggest a link between thyroid hormones pharmacology and athletes' health based on current consumption trends in a population of athletes.

METHODS

Methods used were observation, description, and synthesis, mainly. Among the documents reviewed were: books, scientific articles, and review articles peer-reviewed. The review covered sources published in the period 1961 to 2021. Only references with a traceable origin were accepted (DOI numbering, ISSN and ISBN, as well as peer-reviewed journals). The data on the consumption of thyroid hormones derivatives were extracted from the Doping Control Forms of athlete samples received at Laboratorio Antidoping FMSI of Rome from 2017 to 2021.

RESULTS

An overview of the biosynthesis, pharmacology, and metabolism of thyroid hormones, including thyronamines and thyronacetic acids, was presented. Likewise, a summary is presented on the relationship between thyroid hormones and ethnic and gender differences, their physiology in sport, and the reasons why their use could be considered attractive for athletes.

CONCLUSION

Today, thyroid hormones are not listed as a prohibited substance by the World Anti-Doping Agency. However, several requests to include levothyroxine on the prohibited list are documented. The observation that the number of athletes taking thyroid hormones is growing, particularly in sports such as cycling, triathlons, and skating, should prompt an update on this topic.

Thyroid hormone regulates glutamine metabolism and anaplerotic fluxes by inducing mitochondrial glutamate aminotransferase GPT2

Thyroid hormones (THs) are key metabolic regulators coordinating short- and long-term energy needs. In skeletal muscle, THs modulate energy metabolism in pathophysiological conditions. Indeed, hypo- and hyperthyroidism are leading causes of muscle weakness and strength; however, the metabolic pathways underlying these effects are still poorly understood. Using molecular, biochemical, and isotope-tracing approaches combined with mass spectrometry and denervation experiments, we find that THs regulate glutamine metabolism and anaplerotic fluxes by up-regulating the glutamate pyruvate transaminase 2 (GPT2) gene. In humans, GPT2 autosomal recessive mutations cause a neurological syndrome characterized by intellectual disability, microcephaly, and progressive motor symptoms. Here, we demonstrate a role of the TH/GPT2 axis in skeletal muscle in which it regulates muscle weight and fiber diameter in resting and atrophic conditions and results in protection from muscle loss during atrophy. These results describe an anabolic route by which THs rewire glutamine metabolism toward the maintenance of muscle mass.

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THs induce the expression of the mitochondrial GPT2 gene in skeletal muscle

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The GPT2 up-regulation by THs enhances anaplerotic cycles and α-KG production

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GPT2 is reduced during muscle atrophy and is reactivated by THs treatment

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GPT2 KO^+/– mice undergo muscle loss that is partially attenuated by THs

L-Thyroxine Improves Vestibular Compensation in a Rat Model of Acute Peripheral Vestibulopathy: Cellular and Behavioral Aspects

Unilateral vestibular lesions induce a vestibular syndrome, which recovers over time due to vestibular compensation. The therapeutic effect of L-Thyroxine (L-T4) on vestibular compensation was investigated by behavioral testing and immunohistochemical analysis in a rat model of unilateral vestibular neurectomy (UVN). We demonstrated that a short-term L-T4 treatment reduced the vestibular syndrome and significantly promoted vestibular compensation. Thyroid hormone receptors (TRα and TRβ) and type II iodothyronine deiodinase (DIO2) were present in the vestibular nuclei (VN), supporting a local action of L-T4. We confirmed the T4-induced metabolic effects by demonstrating an increase in the number of cytochrome oxidase-labeled neurons in the VN three days after the lesion. L-T4 treatment modulated glial reaction by decreasing both microglia and oligodendrocytes in the deafferented VN three days after UVN and increased cell proliferation. Survival of newly generated cells in the deafferented vestibular nuclei was not affected, but microglial rather than neuronal differentiation was favored by L-T4 treatment.

Factors mediating spaceflight-induced skeletal muscle atrophy

Skeletal muscle atrophy is a well-known consequence of spaceflight. Because of the potential significant impact of muscle atrophy and muscle dysfunction on astronauts and to their mission, a thorough understanding of the mechanisms of this atrophy and the development of effective countermeasures is critical. Spaceflight-induced muscle atrophy is similar to atrophy seen in many terrestrial conditions, and therefore our understanding of this form of atrophy may also contribute to the treatment of atrophy in humans on Earth. The unique environmental features humans encounter in space include the weightlessness of microgravity, space radiation, and the distinctive aspects of living in a spacecraft. The disuse and unloading of muscles in microgravity are likely the most significant factors that mediate spaceflight-induced muscle atrophy, and have been extensively studied and reviewed. However, there are numerous other direct and indirect effects on skeletal muscle that may be contributing factors to the muscle atrophy and dysfunction seen as a result of spaceflight. This review offers a novel perspective on the issue of muscle atrophy in space by providing a comprehensive overview of the unique aspects of the spaceflight environment and the various ways in which they can lead to muscle atrophy. We systematically review the potential contributions of these different mechanisms of spaceflight-induced atrophy and include findings from both actual spaceflight and ground-based models of spaceflight in humans, animals, and in vitro studies.

Immunometabolic signatures predict recovery from thyrotoxic myopathy in patients with Graves' disease

BACKGROUND

Thyroid hormone excess induces protein energy wasting, which in turn promotes muscle weakness and bone loss in patients with Graves' disease. Although most studies have confirmed a relationship between thyrotoxicosis and muscle dysfunction, few have measured changes in plasma metabolites and immune cells during the development and recovery from thyrotoxic myopathy. The aim of this study was to identify specific plasma metabolites and T-cell subsets that predict thyrotoxic myopathy recovery in patients with Graves' disease.

METHODS

One hundred patients (mean age, 40.0 ± 14.2 years; 67.0% female), with newly diagnosed or relapsed Graves' disease were enrolled at the start of methimazole treatment. Handgrip strength and Five Times Sit to Stand Test performance time were measured at Weeks 0, 12, and 24. In an additional 35 patients (mean age, 38.9 ± 13.5 years; 65.7% female), plasma metabolites and immunophenotypes of peripheral blood were evaluated at Weeks 0 and 12, and the results of a short physical performance battery assessment were recorded at the same time.

RESULTS

In both patient groups, methimazole-induced euthyroidism was associated with improved handgrip strength and lower limb muscle function at 12 weeks. Elevated plasma metabolites including acylcarnitines were restored to normal levels at Week 12 regardless of gender, body mass index, or age (P trend <0.01). Senescent CD8⁺ CD28^- CD57⁺ T-cell levels in peripheral blood were positively correlated with acylcarnitine levels (P < 0.05) and decreased during thyrotoxicosis recovery (P < 0.05). High levels of senescent CD8⁺ T cells at Week 0 were significantly associated with small increases in handgrip strength after 12 weeks of methimazole treatment (P < 0.05), but not statistically associated with Five Times Sit to Stand Test performance.

CONCLUSIONS

Restoring euthyroidism in Graves' disease patients was associated with improved skeletal muscle function and performance, while thyroid hormone-associated changes in plasma acylcarnitines levels correlated with muscle dysfunction recovery. T-cell senescence-related systemic inflammation correlated with plasma acylcarnitine levels and was also associated with small increases in handgrip strength.

A Low Free T3 to Free T4 Ratio Is Associated with Sarcopenia in Euthyroid Patients with Type 2 Diabetes Mellitus

BACKGROUND

This research evaluated the link between normal thyroid hormone levels and sarcopenia in patients with type 2 diabetes mellitus (T2DM).

METHODS

This cross-sectional study enrolled 312 euthyroid patients with T2DM from Qilu Hospital of the Shandong University, China. Body composition, grip strength, and physical performance were assessed as per the 2019 consensus guidelines of the Asian Working Group for Sarcopenia. Binary logistic regression was used to examine the correlation between thyroid hormone levels and sarcopenia and its components.

RESULTS

The prevalence of sarcopenia was 26.9%. Following adjustments for potential confounders, a high-normal serum free triiodothyronine (FT3) level (odds ratio (OR) = 0.522, 95% confidence interval (CI): 0.304-0.895, P = 0.018), a low-normal serum free thyroxine (FT4) level (OR = 1.126, 95% CI: 1.009-1.258, P = 0.034), and a heightened FT3/FT4 ratio (OR = 0.923, 95% CI: 0.879-0.969, P = 0.001) were linked to a low prevalence of sarcopenia. Considering the components of sarcopenia, FT3 concentration was positively associated with muscle strength (OR = 0.525, 95% CI: 0.305-0.902, P = 0.020) and physical performance (OR = 0.443, 95% CI: 0.259-0.758, P = 0.003), while FT4 concentration was negatively linked to muscle mass (OR = 1.114, 95% CI: 1.009-1.232, P = 0.036). The FT3/FT4 ratio was positively linked to muscle mass (OR = 0.943, 95% CI: 0.905-0.981, P = 0.006), muscle strength (OR = 0.945, 95% CI: 0.901-0.992, P = 0.021), and physical performance (OR = 0.934, 95% CI: 0.894-0.975, P = 0.002). Nevertheless, thyroid-stimulating hormone concentration was not associated with sarcopenia.

CONCLUSION

A high FT3/FT4 ratio was significantly linked to a lowered risk of sarcopenia in euthyroid patients with T2DM.

Is constitutional thinness really different from anorexia nervosa? A systematic review and meta-analysis

A growing interest in constitutional thinness has been observed in the last decades, but the publications however cover various fields of study and report equivocal results. The present work systematically reviewed any clinical trials enrolling participants with constitutional thinness and bibliographic researches were performed between December 2018 and June 2020. From a total of 1 212 records initially identified, 402 records were removed as duplicates, 381 articles were excluded based on titles or abstracts and 390 references were excluded against eligibility criteria. Thirty-nine articles were finally included in the systematic review. The results showed that constitutionally thin people seem to be underweight but not underfat and present a fat-free mass as blunted as anorexic patients, despite being a little less underweight. The meta-analysis confirmed that constitutionally thin people present normal energy intake and revealed a trend toward a higher resting metabolic rate to fat-free mass ratio which suggests a highly metabolic fat-free mass. Contrary to patients with anorexia nervosa, constitutionally thin people present normal levels of insulin-like growth factor 1, estradiol, growth hormone, follicle-stimulating hormone, and luteinizing hormone. An intermediate level of leptin between anorexic and control participants was however observed in constitutional thinness. While all the studies reported normal free triiodothyronine and cortisol levels in constitutionally thin individuals, a higher fasting free triiodothyronine level (p = 0.033) and a lower 24 h mean cortisol level (p = 0.005) were observed for the first time. Present results give robust evidence that constitutionally thin people present an atypical phenotype highly different from anorexia nervosa.

Six1 promotes skeletal muscle thyroid hormone response through regulation of the MCT10 transporter

BACKGROUND

The Six1 transcription factor is implicated in controlling the development of several tissue types, notably skeletal muscle. Six1 also contributes to muscle metabolism and its activity is associated with the fast-twitch, glycolytic phenotype. Six1 regulates the expression of certain genes of the fast muscle program by directly stimulating their transcription or indirectly acting through a long non-coding RNA. We hypothesized that additional mechanisms of action of Six1 might be at play.

METHODS

A combined analysis of gene expression profiling and genome-wide location analysis data was performed. Results were validated using in vivo RNA interference loss-of-function assays followed by measurement of gene expression by RT-PCR and transcriptional reporter assays.

RESULTS

The Slc16a10 gene, encoding the thyroid hormone transmembrane transporter MCT10, was identified as a gene with a transcriptional enhancer directly bound by Six1 and requiring Six1 activity for full expression in adult mouse tibialis anterior, a predominantly fast-twitch muscle. Of the various thyroid hormone transporters, MCT10 mRNA was found to be the most abundant in skeletal muscle, and to have a stronger expression in fast-twitch compared to slow-twitch muscle groups. Loss-of-function of MCT10 in the tibialis anterior recapitulated the effect of Six1 on the expression of fast-twitch muscle genes and led to lower activity of a thyroid hormone receptor-dependent reporter gene.

CONCLUSIONS

These results shed light on the molecular mechanisms controlling the tissue expression profile of MCT10 and identify modulation of the thyroid hormone signaling pathway as an additional mechanism by which Six1 influences skeletal muscle metabolism.

Thyroid Hormone Action in Muscle Atrophy

Skeletal muscle atrophy is a condition associated with various physiological and pathophysiological conditions, such as denervation, cachexia, and fasting. It is characterized by an altered protein turnover in which the rate of protein degradation exceeds the rate of protein synthesis, leading to substantial muscle mass loss and weakness. Muscle protein breakdown reflects the activation of multiple proteolytic mechanisms, including lysosomal degradation, apoptosis, and ubiquitin-proteasome. Thyroid hormone (TH) plays a key role in these conditions. Indeed, skeletal muscle is among the principal TH target tissue, where TH regulates proliferation, metabolism, differentiation, homeostasis, and growth. In physiological conditions, TH stimulates both protein synthesis and degradation, and an alteration in TH levels is often responsible for a specific myopathy. Intracellular TH concentrations are modulated in skeletal muscle by a family of enzymes named deiodinases; in particular, in muscle, deiodinases type 2 (D2) and type 3 (D3) are both present. D2 activates the prohormone T4 into the active form triiodothyronine (T3), whereas D3 inactivates both T4 and T3 by the removal of an inner ring iodine. Here we will review the present knowledge of TH action in skeletal muscle atrophy, in particular, on the molecular mechanisms presiding over the control of intracellular T3 concentration in wasting muscle conditions. Finally, we will discuss the possibility of exploiting the modulation of deiodinases as a possible therapeutic approach to treat muscle atrophy.

Smoothelin-Like Protein 1 Regulates Development and Metabolic Transformation of Skeletal Muscle in Hyperthyroidism

Hyperthyroidism triggers a glycolytic shift in skeletal muscle (SKM) by altering the expression of metabolic proteins, which is often accompanied by peripheral insulin resistance. Our previous results show that smoothelin-like protein 1 (SMTNL1), a transcriptional co-regulator, promotes insulin sensitivity in SKM. Our aim was to elucidate the role of SMTNL1 in SKM under physiological and pathological 3,3',5-Triiodo-L-thyronine (T3) concentrations. Human hyper- and euthyroid SKM biopsies were used for microarray analysis and proteome profiler arrays. Expression of genes related to energy production, nucleic acid- and lipid metabolism was changed significantly in hyperthyroid samples. The phosphorylation levels and activity of AMPKα2 and JNK were increased by 15% and 23%, respectively, in the hyperthyroid samples compared to control. Moreover, SMTNL1 expression showed a 6-fold decrease in the hyperthyroid samples and in T3-treated C2C12 cells. Physiological and supraphysiological concentrations of T3 were applied on differentiated C2C12 cells upon SMTNL1 overexpression to assess the activity and expression level of the elements of thyroid hormone signaling, insulin signaling and glucose metabolism. Our results demonstrate that SMTNL1 selectively regulated TRα expression. Overexpression of SMTNL1 induced insulin sensitivity through the inhibition of JNK activity by 40% and hampered the non-genomic effects of T3 by decreasing the activity of ERK1/2 through PKCδ. SMTNL1 overexpression reduced IRS1 Ser307 and Ser612 phosphorylation by 52% and 53%, respectively, in hyperthyroid model to restore the normal responsiveness of glucose transport to insulin. SMTNL1 regulated glucose phosphorylation and balances glycolysis and glycogen synthesis via the downregulation of hexokinase II by 1.3-fold. Additionally, mitochondrial respiration and glycolysis were measured by SeaHorse analysis to determine cellular metabolic function/phenotype of our model system in real-time. T3 overload strongly increased the rate of acidification and a shift to glycolysis, while SMTNL1 overexpression antagonizes the T3 effects. These lines of evidence suggest that SMTNL1 potentially prevents hyperthyroidism-induced changes in SKM, and it holds great promise as a novel therapeutic target in insulin resistance.

Selective Inhibition of Genomic and Non-Genomic Effects of Thyroid Hormone Regulates Muscle Cell Differentiation and Metabolic Behavior

Thyroid hormones (THs) are key regulators of different biological processes. Their action involves genomic and non-genomic mechanisms, which together mediate the final effects of TH in target tissues. However, the proportion of the two processes and their contribution to the TH-mediated effects are still poorly understood. Skeletal muscle is a classical target tissue for TH, which regulates muscle strength and contraction, as well as energetic metabolism of myofibers. Here we address the different contribution of genomic and non-genomic action of TH in skeletal muscle cells by specifically silencing the deiodinase Dio2 or the β3-Integrin expression via CRISPR/Cas9 technology. We found that myoblast proliferation is inversely regulated by integrin signal and the D2-dependent TH activation. Similarly, inhibition of the nuclear receptor action reduced myoblast proliferation, confirming that genomic action of TH attenuates proliferative rates. Contrarily, genomic and non-genomic signals promote muscle differentiation and the regulation of the redox state. Taken together, our data reveal that integration of genomic and non-genomic signal pathways finely regulates skeletal muscle physiology. These findings not only contribute to the understanding of the mechanisms involved in TH modulation of muscle physiology but also add insight into the interplay between different mechanisms of action of TH in muscle cells.

The Association of TSH and Thyroid Hormones With Lymphopenia in Bacterial Sepsis and COVID-19

CONTEXT

Lymphopenia is a key feature of immune dysfunction in patients with bacterial sepsis and coronavirus disease 2019 (COVID-19) and is associated with poor clinical outcomes, but the cause is largely unknown. Severely ill patients may present with thyroid function abnormalities, so-called nonthyroidal illness syndrome, and several studies have linked thyrotropin (thyroid stimulating hormone, TSH) and the thyroid hormones thyroxine (T4) and 3,5,3'-triiodothyronine (T3) to homeostatic regulation and function of lymphocyte populations.

OBJECTIVE

This work aimed to test the hypothesis that abnormal thyroid function correlates with lymphopenia in patients with severe infections.

METHODS

A retrospective analysis of absolute lymphocyte counts, circulating TSH, T4, free T4 (FT4), T3, albumin, and inflammatory biomarkers was performed in 2 independent hospitalized study populations: bacterial sepsis (n = 224) and COVID-19 patients (n = 161). A subgroup analysis was performed in patients with severe lymphopenia and normal lymphocyte counts.

RESULTS

Only T3 significantly correlated (ρ = 0.252) with lymphocyte counts in patients with bacterial sepsis, and lower concentrations were found in severe lymphopenic compared to nonlymphopenic patients (n = 56 per group). Severe lymphopenic COVID-19 patients (n = 17) showed significantly lower plasma concentrations of TSH, T4, FT4, and T3 compared to patients without lymphopenia (n = 18), and demonstrated significantly increased values of the inflammatory markers interleukin-6, C-reactive protein, and ferritin. Remarkably, after 1 week of follow-up, the majority (12 of 15) of COVID-19 patients showed quantitative recovery of their lymphocyte numbers, whereas TSH and thyroid hormones remained mainly disturbed.

CONCLUSION

Abnormal thyroid function correlates with lymphopenia in patients with severe infections, like bacterial sepsis and COVID-19, but future studies need to establish whether a causal relationship is involved.

Thyroid Hormone Enhances Angiogenesis and the Warburg Effect in Squamous Cell Carcinomas

Simple Summary

Cancer cells rewire their metabolism to promote growth, survival, proliferation, and long-term maintenance. Aerobic glycolysis is a prominent trait of many cancers; contextually, glutamine addiction, enhanced glucose uptake and aerobic glycolysis sustain the metabolic needs of rapidly proliferating cancer cells. Thyroid hormone (TH) is a positive regulator of tumor progression and metastatic conversion of squamous cell carcinoma (SCC). Accordingly, overexpression of the TH activating enzyme, D2, is associated with metastatic SCC. The aim of our study was to assess the ability of TH and its activating enzyme in promoting key tracts of cancer progression such as angiogenesis, response to hypoxia and metabolic adaptation. By performing in vivo and in vitro studies, we demonstrate that TH induces VEGF-A in cancer cells and fosters aerobic glycolysis inducing pro-glycolytic mediators, thus implying that TH signal attenuation represents a therapeutic tool to contrast tumor angiogenesis and tumor progression.

Abstract

Cancer angiogenesis is required to support energetic demand and metabolic stress, particularly during conditions of hypoxia. Coupled to neo-vasculogenesis, cancer cells rewire metabolic programs to sustain growth, survival and long-term maintenance. Thyroid hormone (TH) signaling regulates growth and differentiation in a variety of cell types and tissues, thus modulating hyper proliferative processes such as cancer. Herein, we report that TH coordinates a global program of metabolic reprogramming and induces angiogenesis through up-regulation of the VEGF-A gene, which results in the enhanced proliferation of tumor endothelial cells. In vivo conditional depletion of the TH activating enzyme in a mouse model of cutaneous squamous cell carcinoma (SCC) reduces the concentration of TH in the tumoral cells and results in impaired VEGF-A production and attenuated angiogenesis. In addition, we found that TH induces the expression of the glycolytic genes and fosters lactate production, which are key traits of the Warburg effect. Taken together, our results reveal a TH–VEGF-A–HIF1α regulatory axis leading to enhanced angiogenesis and glycolytic flux, which may represent a target for SCC therapy.

Body Weight Changes in Hyperthyroidism: Timing and Possible Explanations during a One Year Repeated Measurement Study

BACKGROUND

Weight gain during treatment of hyperthyroidism is a frequent and for many patients unwanted outcome. With this repeated measurement study, we explored the timing of weight changes during the first year of antithyroid drug (ATD) treatment and assessed the correlation between body weight changes and changes in thyroid hormones, resting energy expenditure (REE), physical activity level, and energy efficiency.

METHODS

Patients with new onset hyperthyroidism were investigated every second month during the first year of ATD treatment. At each investigation, the following were measured: body weight, thyroid hormone concentrations, physical activity level, and daily number of steps, REE, and exercise performance.

RESULTS

Two men and eleven women, all sedentary, mean age 49(SD: 9.3) years were included. Significant changes after 1 year occurred for body weight (68.9-74.1 kg), thyroid hormones (free T3 [fT3] 17.5 to 4.42 pmol/L), REE (1,630-1,484 kcal/24 h), and energy efficiency at lower (50 W) workloads (16.0-17.6%). In individual patients, only REE and fT3 correlated to changes in body weight. Physical activity level did not change during treatment.

CONCLUSION

In this study, treatment of hyperthyroidism was associated with marked increase in body weight in the patients. This increase correlated to a decrease in REE and only to a negligible extent to changes in energy efficiency and not at all to changes in physical activity level of daily living.

Nutrient Intake and Muscle Measures in Geriatric Outpatients

OBJECTIVE

Low muscle mass and muscle function are associated with adverse health outcomes in older adults. This study examined nutrient intake as a potential contributing factor for low muscle mass, muscle strength, and muscle power in geriatric outpatients.

METHOD

This cross-sectional study included geriatric outpatients (n = 58, 38 female) with a mean age of 77.2 ± 9.0 years referred to the Falls and Balance outpatient clinic between December 2017 and January 2019. Nutrient intake (macro- and micronutrients) was examined using a 3-day food diary. Energy-adjusted nutrient intake was calculated using the residual method. Sex-standardized muscle measures included muscle mass assessed using bioelectrical impedance analysis (skeletal muscle mass [SMM in kilograms], SMM index [SMM/height² in kg/m²], and SMM/body mass index), handgrip strength (muscle strength) assessed using a dynamometer, and chair-stand test (muscle power). Univariate linear regression analyses were used to examine the associations of nutrient intake with muscle measures adjusted for age and body weight. A Bonferroni correction was applied to account for multiple testing (p < 0.001).

RESULTS

Higher energy, iodine, and folate intake were associated with higher muscle mass, and higher folate intake was associated with higher muscle strength (p < 0.05). After Bonferroni correction, none of the nutrient intakes remained statistically significant. None of the other nutrients was associated with muscle measures.

CONCLUSIONS

Only a few nutrients were associated with muscle measures. Nutrient intake appears to be more related to muscle mass than muscle strength and muscle power in geriatric outpatients.

Mechanism of thyroid hormone signaling in skeletal muscle of aging mice

BACKGROUND AND AIM

Skeletal muscle (SM) has been shown as a target of thyroid hormones (THs). However, the status of TH signaling in aged SM remains unclear. This study aimed to explore the mechanism of TH signaling in SM of aging mice.

METHODS

Thirty C57BL/6J male mice were divided into 6-, 15- and 22-month (6, 15 and 22M) groups according to different age. Physical parameters were evaluated by analytical balance, grip strength test and histological analysis. Thyroid function was detected by enzyme-linked immunosorbent assay. TH signaling was compared among the three groups by real-time PCR and western blotting analysis.

RESULTS

p16, p21, and p53 mRNA levels in SM increased in age-dependent manner. The muscle weight and strength decreased in 22M group compared to 6 and 15M groups. Concentrations of thyroid hormones, including free triiodothyronine (FT3), free thyroxine (FT4) and thyroid-stimulating hormone (TSH) in 22 M mice were not shown significant difference compared to 6M or 15M mice, although FT3 showed slightly decrease and TSH appeared a mild increase accompanying with age. mRNA levels of TH transporters, including MCT8 and MCT10, as well as iodothyronine deiodinase type 2 (DIO2) and type 3 (DIO3), were higher in 22M, while TH receptor α (TRα) mRNA and protein expression was lower in 22M, compared to the other groups. Type-I myosin heavy chain (MyHC I), MyHC IIx, and MyHC IIa were upregulated and Type-IIb MyHC (MyHC IIb) was downregulated in SM with advancing age.

CONCLUSIONS

TH signaling in SM changes with aging.

Metabolic and Energy Imbalance in Dysglycemia-Based Chronic Disease

Metabolic flexibility is the ability to efficiently adapt metabolism based on nutrient availability and requirement that is essential to maintain homeostasis in times of either caloric excess or restriction and during the energy-demanding state. This regulation is orchestrated in multiple organ systems by the alliance of numerous metabolic pathways under the master control of the insulin-glucagon-sympathetic neuro-endocrine axis. This, in turn, regulates key metabolic enzymes and transcription factors, many of which interact closely with and culminate in the mitochondrial energy generation machinery. Metabolic flexibility is compromised due to the continuous mismatch between availability and intake of calorie-dense foods and reduced metabolic demand due to sedentary lifestyle and age-related metabolic slowdown. The resultant nutrient overload leads to mitochondrial trafficking of substrates manifesting as mitochondrial dysfunction characterized by ineffective substrate switching and incomplete substrate utilization. At the systemic level, the manifestation of metabolic inflexibility comprises reduced skeletal muscle glucose disposal rate, impaired suppression of hepatic gluconeogenesis and adipose tissue lipolysis manifesting as insulin resistance. This is compounded by impaired β-cell function and progressively reduced β-cell mass. A consequence of insulin resistance is the upregulation of the mitogen-activated protein kinase pathway leading to a pro-hypertensive, atherogenic, and thrombogenic environment. This is further aggravated by oxidative stress, advanced glycation end products, and inflammation, which potentiates the risk of micro- and macro-vascular complications. This review aims to elucidate underlying mechanisms mediating the onset of metabolic inflexibility operating at the main target organs and to understand the progression of metabolic diseases. This could potentially translate into a pharmacological tool that can manage multiple interlinked conditions of dysglycemia, hypertension, and dyslipidemia by restoring metabolic flexibility. We discuss the breadth and depth of metabolic flexibility and its impact on health and disease.

The Expression of Hormone Receptors as a Gateway toward Understanding Endocrine Actions in Female Pelvic Floor Muscles

OBJECTIVE

To provide an overview of the hormone actions and receptors expressed in the female pelvic floor muscles, relevant for understanding the pelvic floor disorders.

METHODS

We performed a literature review focused on the expression of hormone receptors mainly in the pelvic floor muscles of women and female rats and rabbits.

RESULTS

The impairment of the pelvic floor muscles can lead to the onset of pelvic floor dysfunctions, including stress urinary incontinence in women. Hormone milieu is associated with the structure and function alterations of pelvic floor muscles, a notion supported by the fact that these muscles express different hormone receptors. Nuclear receptors, such as steroid receptors, are up till now the most investigated. The present review accounts for the limited studies conducted to elucidate the expression of hormone receptors in pelvic floor muscles in females.

CONCLUSION

Hormone receptor expression is the cornerstone in some hormone-based therapies, which require further detailed studies on the distribution of receptors in particular pelvic floor muscles, as well as their association with muscle effectors, involved in the alterations relevant for understanding pelvic floor disorders.

Adaptive Thermogenesis Driving Catch-Up Fat Is Associated With Increased Muscle Type 3 and Decreased Hepatic Type 1 Iodothyronine Deiodinase Activities: A Functional and Proteomic Study

Refeeding after caloric restriction induces weight regain and a disproportionate recovering of fat mass rather than lean mass (catch-up fat) that, in humans, associates with higher risks to develop chronic dysmetabolism. Studies in a well-established rat model of semistarvation-refeeding have reported that catch-up fat associates with hyperinsulinemia, glucose redistribution from skeletal muscle to white adipose tissue and suppressed adaptive thermogenesis sustaining a high efficiency for fat deposition. The skeletal muscle of catch-up fat animals exhibits reduced insulin-stimulated glucose utilization, mitochondrial dysfunction, delayed in vivo contraction-relaxation kinetics, increased proportion of slow fibers and altered local thyroid hormone metabolism, with suggestions of a role for iodothyronine deiodinases. To obtain novel insights into the skeletal muscle response during catch-up fat in this rat model, the functional proteomes of tibialis anterior and soleus muscles, harvested after 2 weeks of caloric restriction and 1 week of refeeding, were studied. Furthermore, to assess the implication of thyroid hormone metabolism in catch-up fat, circulatory thyroid hormones as well as liver type 1 (D1) and liver and skeletal muscle type 3 (D3) iodothyronine deiodinase activities were evaluated. The proteomic profiling of both skeletal muscles indicated catch-up fat-induced alterations, reflecting metabolic and contractile adjustments in soleus muscle and changes in glucose utilization and oxidative stress in tibialis anterior muscle. In response to caloric restriction, D3 activity increased in both liver and skeletal muscle, and persisted only in skeletal muscle upon refeeding. In parallel, liver D1 activity decreased during caloric restriction, and persisted during catch-up fat at a time-point when circulating levels of T4, T3 and rT3 were all restored to those of controls. Thus, during catch-up fat, a local hypothyroidism may occur in liver and skeletal muscle despite systemic euthyroidism. The resulting reduced tissue thyroid hormone bioavailability, likely D1- and D3-dependent in liver and skeletal muscle, respectively, may be part of the adaptive thermogenesis sustaining catch-up fat. These results open new perspectives in understanding the metabolic processes associated with the high efficiency of body fat recovery after caloric restriction, revealing new implications for iodothyronine deiodinases as putative biological brakes contributing in suppressed thermogenesis driving catch-up fat during weight regain.

Irisin as a Potential Biomarker Associated with Myocardial Injuries in Patients with Severe Hypothyroidism

OBJECTIVE

Irisin, a novel myokine, has recently been considered to produce a cardioprotective effect. Potential biomarkers for myocardial injuries in patients with severe hypothyroidism have yet to be identified. We aimed to investigate whether serum irisin may serve as a promising biomarker for early detecting the myocardial injuries in patients with severe hypothyroidism.

METHODS

This cross-sectional study comprised 25 newly diagnosed drug-naive patients with severe primary hypothyroidism and 17 age- and sex-matched healthy controls. Circulating irisin levels and cardiac magnetic resonance (CMR) were evaluated in each participant. Left ventricular (LV) myocardial injuries were detected by CMR-based T1 mapping technique using a modified look-locker inversion recovery (MOLLI) sequence, which is quantified as native T1 values.

RESULTS

Compared with healthy controls, the severe hypothyroidism group had significantly lower levels of serum irisin, especially those with pericardial effusion (P < 0.05). The severe hypothyroidism subjects exhibited lower peak filling rates (PFRs) and higher native myocardial T1 values than controls (P < 0.05). Additionally, the ROC analysis displayed that the sensitivity and specificity of serum irisin for diagnosing pericardial effusion in patients with severe hypothyroidism were 73.3% and 100.0%, respectively. The AUC was 0.920 (0.861-1.000) (P < 0.001). The cutoff value was 36.94 ng/mL. Moreover, the results in subgroup analysis revealed that the native T1 values of the low-irisin group were significantly higher than that of the high-irisin group (P < 0.05). According to multivariate linear regression analysis, serum irisin concentrations were negatively and independently correlated with native myocardial T1 values after adjustment for age, sex, and other conventional confounding factors (β = -1.473, P < 0.05).

CONCLUSIONS

Irisin may be a potential biomarker for predicting myocardial injuries in patients with severe hypothyroidism.

The correlation between serum thyroid hormone levels and hand grip among elderly male Chinese inpatients

INTRODUCTION

Thyroid dysfunction is closely associated with skeletal muscle weakness. However, data on the optimal serum range of thyroid hormones for maintaining muscle strength in the elderly is lacking.

METHODS

We conducted a cross-sectional analysis in male elderly inpatients from the Geriatric Department of Zhongshan Hospital (affiliated to Fudan University, Shanghai, China). Serum biochemical parameters and thyroid hormones were detected for each participant. Hand grip (HG) was measured, with low hand grip defined as HG <26 kg according to the standard of the Asian Working Group for Sarcopenia. Logistic regression was used to evaluate the effects of different serum thyroid hormone levels on HG.

RESULTS

The majority of the subjects were euthyroid. The prevalence of low hand grip was 48.5%. Stratified by the free thyroxine (FT4) quartiles, the results showed HG was the highest in the third quartile. Multiple logistic regression analysis showed that compared with those in the first quartile, subjects in the third quartile of FT4 had a significantly lower risk of low hand grip (OR = 0.133, 95%CI: 0.020-0.610, p = .009), after adjusting potential confounding factors.

CONCLUSION

In elderly male inpatients, maintaining a narrower serum range of thyroid hormone might be needed to protect skeletal muscle strength.

Thyroid hormone receptor α in skeletal muscle is essential for T3-mediated increase in energy expenditure

Thyroid hormones are important for homeostatic control of energy metabolism and body temperature. Although skeletal muscle is considered a key site for thyroid action, the contribution of thyroid hormone receptor signaling in muscle to whole‐body energy metabolism and body temperature has not been resolved. Here, we show that T3‐induced increase in energy expenditure requires thyroid hormone receptor alpha 1 (TRα₁) in skeletal muscle, but that T3‐mediated elevation in body temperature is achieved in the absence of muscle‐TRα₁. In slow‐twitch soleus muscle, loss‐of‐function of TRα₁ (TRα^HSACre) alters the fiber‐type composition toward a more oxidative phenotype. The change in fiber‐type composition, however, does not influence the running capacity or motivation to run. RNA‐sequencing of soleus muscle from WT mice and TRα^HSACre mice revealed differentiated transcriptional regulation of genes associated with muscle thermogenesis, such as sarcolipin and UCP3, providing molecular clues pertaining to the mechanistic underpinnings of TRα₁‐linked control of whole‐body metabolic rate. Together, this work establishes a fundamental role for skeletal muscle in T3‐stimulated increase in whole‐body energy expenditure.

Upper and Lower Limb Strength and Body Posture in Children with Congenital Hypothyroidism: An Observational Case-Control Study

BACKGROUND

Congenital hypothyroidism (CH) is an endocrine disease with a precocious significant impairment of growth and neuromotor development. Thyroid hormones are essential for central nervous system development, maturation, and myelination. Furthermore, thyroid hormone deficiency affects the function of several systems, including the musculoskeletal system. The disease has a significant incidence in the general population (1:3000-1:2000 newborns in Italy). The aim of the present study was to evaluate any differences in upper and lower limb strength, body sway, and plantar loading distribution in children with CH compared to healthy children.

METHODS

In this study, the case group was composed of children with CH (CHG), while the control group included healthy children (CG). Both groups comprised 19 children (CHG: female = 12; CG: female = 9). The maximum isometric handgrip strength and explosive-elastic lower limb strength were assessed with the handgrip test and the Sargent test, respectively. The stabilometric and baropodometric analyses were used to measure the Center of Pressure displacements and the plantar loading distribution between feet, respectively. The differences between groups were analyzed by a univariate analysis of covariance using as covariates weight and height with the significant level set at < 0.05.

RESULTS

We found that CHG children were shorter and thinner than CG ones (p < 0.05). No significant difference in the upper and lower limb strength was found between groups. CHG exhibited a significant greater Sway Path Length (p < 0.01) and Ellipse Surface (p < 0.05) than CG. Moreover, CHG displayed an asymmetric plantar loading distribution with a significant lower percentage in the right than in the left foot (p < 0.05). Moreover, a significant lower plantar loading percentage in the right foot of CHG than in the right foot of CG was observed (p < 0.05).

CONCLUSIONS

These findings seem to suggest that CH does not affect muscle strength in early treated children. However, these patients show poor postural control ability and asymmetric plantar loading distribution. Increasing the physical activity in these children could improve their body posture.

Disambiguating "Mechanisms" in Pharmacy: Lessons from Mechanist Philosophy of Science

Talk of mechanisms is ubiquitous in the natural sciences. Interdisciplinary fields such as biochemistry and pharmacy frequently discuss mechanisms with the assistance of diagrams. Such diagrams usually depict entities as structures or boxes and activities or interactions as arrows. While some of these arrows may indicate causal or componential relations, others may represent temporal or operational orders. Importantly, what kind of relation an arrow represents may not only vary with context but also be underdetermined by empirical data. In this manuscript, we investigate how an analysis of pharmacological mechanisms in terms of producing and underlying mechanisms-as discussed in the contemporary philosophy of science-may shed light on these issues. Specifically, we shall argue that while pharmacokinetic mechanisms usually describe causal chains of production, pharmacodynamics tends to focus on mechanisms of action underlying the in vivo effects of a drug. Considering the action of thyroid gland hormones in the human body as a case study, we further demonstrate that pharmacodynamic schemes tend to incorporate entities and interactions on multiple levels. Yet, traditional pharmacodynamic schemes are sketched "flat", i.e., non-hierarchically. We suggest that transforming flat pharmacodynamic schemes into mechanistic multi-level representations may assist in disentangling the different kinds of mechanisms and relations depicted by arrows in flat schemes. The resulting Baumkuchen model provides a powerful and practical alternative to traditional flat schemes, as it explicates the relevant mechanisms and relations more clearly. On a more general note, our discussion demonstrates how pharmacology and related disciplines may benefit from applying concepts from the new mechanist philosophy to guide the interpretation of scientific diagrams.

A Significant Association of Upper Limb Muscle Strength with Thyroid Function in Overweight and Obese Population: A Study of the Sixth Korea National Health and Nutrition Examination Survey (KNHANES 2014-2015)

Background. As skeletal muscle is one of main targets of thyroid hormone signalling, an association of thyroid function and muscle strength could be expected. The aim of study is to evaluate the association of free thyroxine (FT4) and thyrotropin (TSH) with upper limb muscle strength, measured by hand grip strength, in subjects with normal FT4 from national representative data. The study utilized the sixth edition of the Korea National Health and Nutrition Examination Survey. After exclusion of subjects with FT4 level out of normal range, a history of thyroid disease or cerebral disease, restricted activity, and incomplete data, a total of 3503 were recruited (age range 19-80 years, 51% male). FT4 positively correlated with upper limb muscle strength (β coefficient = -12.84, p < 0.001), while TSH did negatively (β coefficient = -0.37, p=0.002). After adjusting for confounding factors, statistical significance disappeared. However, among subjects with BMI above 23 kg/m², a negative correlation of TSH with upper limb muscle strength was found in a younger age group (19-39 years old) (β coefficient = -0.56, p=0.021), while FT4 positively correlated with upper limb muscle strength (β coefficient = 3.24, p=0.019) in an older group (above 40 years old). In overweight and obese subjects, a significant association of thyroid function with upper limb muscle strength was observed in nation-wide representative data. High TSH in a younger group and low FT4 in an older group could be risk factors for decreased upper limb muscle strength in obese population.

Thyroid Hormone Protects from Fasting-Induced Skeletal Muscle Atrophy by Promoting Metabolic Adaptation

Thyroid hormones regulate a wide range of cellular responses, via non-genomic and genomic actions, depending on cell-specific thyroid hormone transporters, co-repressors, or co-activators. Skeletal muscle has been identified as a direct target of thyroid hormone T3, where it regulates stem cell proliferation and differentiation, as well as myofiber metabolism. However, the effects of T3 in muscle-wasting conditions have not been yet addressed. Being T3 primarily responsible for the regulation of metabolism, we challenged mice with fasting and found that T3 counteracted starvation-induced muscle atrophy. Interestingly, T3 did not prevent the activation of the main catabolic pathways, i.e., the ubiquitin-proteasome or the autophagy-lysosomal systems, nor did it stimulate de novo muscle synthesis in starved muscles. Transcriptome analyses revealed that T3 mainly affected the metabolic processes in starved muscle. Further analyses of myofiber metabolism revealed that T3 prevented the starvation-mediated metabolic shift, thus preserving skeletal muscle mass. Our study elucidated new T3 functions in regulating skeletal muscle homeostasis and metabolism in pathological conditions, opening to new potential therapeutic approaches for the treatment of skeletal muscle atrophy.

Effects of Short-Term Fasting and Different Overfeeding Diets on Thyroid Hormones in Healthy Humans

Background: A greater decrease in 24-hour energy expenditure (EE) during fasting and a smaller increase in 24-hour EE during low-protein overfeeding (metabolic "thrifty" phenotype) predict weight gain. As thyroid hormones (TH) are implicated in energy intake and metabolism, we assessed whether: (i) TH concentrations are altered by 24-hour fasting or overfeeding diets with varying protein content and (ii) diet-related changes in TH correlate with concomitant changes in EE. Methods: Fifty-eight euthyroid healthy subjects with normal glucose regulation underwent 24-hour dietary interventions including fasting, eucaloric feeding, and five overfeeding diets in a crossover design within a whole-room indirect calorimeter to measure the 24-hour EE. Overfeeding diets (200% of energy requirements) included three diets with 20% protein, one diet with 3% protein (low-protein overfeeding diet [LPF]: 46% fat), and one diet with 30% protein (high-protein overfeeding diet [HPF]: 44% fat, n = 51). Plasma free thyroxine (fT4), free triiodothyronine (fT3), and fibroblast growth factor 21 (FGF21) concentrations were measured after overnight fast the morning of and after each diet. Results: On average, fT4 increased by 8% (+0.10 ng/dL, 95% confidence interval [CI 0.07-0.13], p < 0.0001) and fT3 decreased by 6% (-0.17 pg/mL [CI -0.27 to -0.07], p = 0.001) after 24-hour fasting, whereas both fT4 and fT3 decreased by 5% (-0.07 ng/dL [CI -0.11 to -0.04], p < 0.0001) and 4% (-0.14 pg/mL [CI -0.24 to -0.04], p = 0.008) following HPF, respectively. Greater decreases in fT3 after HPF are associated with larger decreases in FGF21 (r = 0.40, p = 0.005). Following LPF, the mean fT3 increased by 6% (+0.14 pg/mL [CI 0.05-0.2], p = 0.003) with no change in fT4 (p = 0.7). No changes in TH were observed after normal-protein overfeeding diets (all p > 0.1). No associations were observed between TH concentrations and diet-related changes in 24-hour EE during any diet (all p > 0.07). Conclusions: Acute (200%) short-term (24 hours) changes in food intake induce small changes in TH concentrations only after diets with low (0% fasting and 3% protein overfeeding) or high (30% protein overfeeding) protein content. The fT3-FGF21 association after high-protein overfeeding suggests a role for TH in inhibiting FGF21 secretion by the liver during protein excess. These results indicate that TH are involved in protein metabolism; however, they do not mediate the short-term EE response to diets that characterize the metabolic phenotypes and determine the individual susceptibility to weight gain.

Deiodinases and their intricate role in thyroid hormone homeostasis

The deiodinase family of enzymes mediates the activation and inactivation of thyroid hormone. The role of these enzymes in the regulation of the systemic concentrations of thyroid hormone is well established and underpins the treatment of common thyroid diseases. Interest in this field has increased in the past 10 years as the deiodinases became implicated in tissue development and homeostasis, as well as in the pathogenesis of a wide range of human diseases. Three deiodinases have been identified, namely, types 1, 2 and 3 iodothyronine deiodinases, which differ in their catalytic properties and tissue distribution. Notably, the expression of these enzymes changes during the lifetime of an individual in relation to the different needs of each organ and to ageing. The systemic homeostatic role of deiodinases clearly emerges during changes in serum concentrations of thyroid hormone, as seen in patients with thyroid dysfunction. By contrast, the role of deiodinases at the tissue level allows thyroid hormone signalling to be finely tuned within a given cell in a precise time-space window without perturbing serum concentrations of thyroid hormone. This Review maps the overall functional role of the deiodinases and explores challenges and novel opportunities arising from the expanding knowledge of these 'master' components of the thyroid homeostatic system.

A Global Loss of Dio2 Leads to Unexpected Changes in Function and Fiber Types of Slow Skeletal Muscle in Male Mice

The type 2 iodothyronine-deiodinase (D2) enzyme converts T4 to T3, and mice deficient in this enzyme [D2 knockout (D2KO) mice] have decreased T3 derived from T4 in skeletal muscle despite normal circulating T3 levels. Because slow skeletal muscle is particularly susceptible to changes in T3 levels, we expected D2 inactivation to result in more pronounced slow-muscle characteristics in the soleus muscle, mirroring hypothyroidism. However, ex vivo studies of D2KO soleus revealed higher rates of twitch contraction and relaxation and reduced resistance to fatigue. Immunostaining of D2KO soleus showed that these properties were associated with changes in muscle fiber type composition, including a marked increase in the number of fast, glycolytic type IIB fibers. D2KO soleus muscle fibers had a larger cross-sectional area, and this correlated with increased myonuclear accretion in myotubes formed from D2KO skeletal muscle precursor cells differentiated in vitro. Consistent with our functional findings, D2KO soleus gene expression was markedly different from that in hypothyroid wild-type (WT) mice. Comparison of gene expression between euthyroid WT and D2KO mice indicated that PGC-1α, a T3-dependent regulator of slow muscle fiber type, was decreased by ∼50% in D2KO soleus. Disruption of Dio2 in the C2C12 myoblast cell line led to a significant decrease in PGC-1α expression and a faster muscle phenotype upon differentiation. These results indicate that D2 loss leads to significant changes in soleus contractile function and fiber type composition that are inconsistent with local hypothyroidism and suggest that reduced levels of PCG-1α may contribute to the observed phenotypical changes.