Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases

Co-exposure to enrofloxacin and atrazine enhances the hepatotoxicity in Larimichthys crocea by targeting the hypothalamic-pituitary-thyroid and gut-liver axes

Enrofloxacin (ENR) and atrazine (ATZ) are common co-contaminants in marine environments. Although the immunosuppressive effects of ENR and the endocrine-disrupting properties of ATZ are well established, the combined effects of these pollutants on hepatotoxicity, particularly concerning the regulation of the hypothalamic-pituitary-thyroid (HPT) and gut-liver axes, remain poorly understood. In this study, Larimichthys crocea was exposed to ENR and ATZ at environmentally relevant concentrations, individually and in combination, to investigate the hepatotoxicity. Liver cell swelling, necrosis, oxidative stress, and elevated liver injury markers were observed, indicating hepatic damage, with co-exposure exacerbating liver injury. Decreased levels of thyrotropin-releasing hormone and thyroid-stimulating hormone, increased triiodothyronine and thyroxine, and altered expression of HPT axis-related genes demonstrated enhanced disruption of the HPT axis under co-exposure, which was strongly associated with oxidative stress and liver dysfunction. Molecular docking confirmed that ENR and ATZ inhibited thyroid hormone binding to target proteins, likely provoking the enhanced hepatotoxicity. Additionally, ATZ significantly intensified the intestinal bacterial disturbances induced by ENR, further aggravating hepatotoxicity through the gut-liver axis. This study is the first to reveal the increased risk associated with ENR and ATZ co-exposure, highlighting the need for attention to such co-contaminants.

Psychological factors and obesity, not thyroid biomarkers, predict thyroid-dependent quality of life in treated hypothyroidism: a cross-sectional study

BACKGROUND

A significant number of patients with primary hypothyroidism report persistent symptoms and decreased quality of life (QoL) despite biochemically adequate levothyroxine replacement. Individual variations in thyroxine conversion, autoimmune inflammation, and psychological factors have all been implicated as a potential cause.

METHODS

In this cross-sectional study we have examined the association of numerous demographic, disease-specific, and laboratory parameters as well as three patient reported outcome measures with thyroid-dependent QoL as measured by the Underactive Thyroid-Dependent Quality of Life Questionnaire. Patients were stringently selected to minimize the confounding effect of comorbidities or inadequate hormone replacement. We used validated questionnaires to assess somatosensory amplification, depression, and symptom number. Determinants of QoL were evaluated using uni- and multivariable linear modeling, and mediation analysis.

RESULTS

Our final sample consisted of 157 patients. 70.7% had Hashimoto's, whereas 29.3% had iatrogenic hypothyroidism. Mean age was 49.5 ± 14.5 years, disease duration: 11.2 ± 8.2 years, thyroxine dose: 1.2 ug/kg bodyweight, TSH: 1.8 ± 0.9 mIU/L. Thyroid-specific biomarkers including TSH, FT3, FT4, rT3, SPINA-GD, anti-TPO, and SHBG had no association with thyroid-dependent QoL. Somatosensory amplification was a strong predictor of the presence and perceived bother of the most common hypothyroidism-associated symptoms. In our final multivariable model (r2 = 0.31) the factors associated with thyroid-dependent QoL were somatosensory amplification (p = 0.002), BMI (p = 0.021), and depression (p < 0.001).

CONCLUSION

These results suggest that psychological factors, particularly somatosensory amplification, might play a major role in influencing QoL in hypothyroid individuals on adequate levothyroxine replacement. Our findings do not corroborate a significant role for autoimmune inflammation or tissue-level hypothyroidism.

Determination of Thyroid Hormones and 11 Metabolites in the Human Serum Using a Simple Derivatization Strategy and Analysis by Isotope-Dilution Liquid Chromatography Tandem Mass Spectrometry

Many analytical methods for thyroid hormone (TH) determination lack sensitivity and/or specificity. The thyroid hormone metabolites (THMs) are usually not measured at all. This study describes the development of sensitive high-throughput analytical methods for determining the total concentration and free fraction of TH and THM in the human serum. For the analysis of the TOTAL fraction, we employed protein precipitation and anionic exchanger solid-phase extraction. For the FREE fraction, ultrafiltration and salt-out liquid partitioning were used. Derivatization using dansyl chloride was employed to enhance the sensitivity of HPLC-ESI-MS/MS analysis. Both protocols were validated according to the European Analytical Guidelines (2002/657/EC). We obtained very good recoveries (73-115%) and precision. Interday coefficients of variation (CVs) for most of the analytes ranged from 1.2 to 16.4%. The sensitivity was excellent with detection limits in the sub ppt range for the majority of TH and THM. A significant enhancement in sensitivity (>10 fold) was achieved through derivatization. The applicability was proved on a set of samples from pregnant women enrolled in the CELSPAC cohort (n = 120). Our TH reference ranges are in good agreement with those reported in the literature. The methods also allowed us to quantify the levels of 11 THM, including some previously undetected THM in total and free fractions, and proved to be suitable for high-throughput routine TH and THM analyses. Our approach offers an important advancement in thyroid hormone analysis. To the best of our knowledge, it is for the first time that data for T1A and T2A as well as for free THM levels in the human serum are published in the literature. Moreover, our study also brings the first information about the levels of most of the THM in pregnant women.

The interplay between thyrotropic axis, neurological complications, and rehabilitation outcomes in patients with traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of mortality and long-term disability, with its pathophysiology encompassing both primary mechanical damage and secondary neuroinflammatory, metabolic, and biochemical alterations. These complex mechanisms contribute to the observed heterogeneous clinical outcomes, including neuroendocrine dysfunctions, post-traumatic seizures, and disorders of consciousness (DoC). Thyroid hormones (THs) play essential roles in synaptic plasticity, neurogenesis and neuronal homeostasis, and the hypothalamic-pituitary-thyroid (HPT) axis has recently emerged as a potential acute and chronic modulator of neurological and functional recovery following TBI, thereby hinting at the potential involvement of THs in post-TBI outcomes. While evidence suggests that alterations in the HPT axis may influence susceptibility to seizures, progression of DoC, and rehabilitation outcomes, an increased blood-brain barrier permeability in concert with dysregulated deiodinase activity and expanding oxidative stress have all been proposed as mechanisms linking THs to post-TBI neurological complications. This review aims to summarize current evidence on the potential role of the thyrotropic axis in neurological and rehabilitation outcomes following TBI.

Reusable Selenenyl Iodide-Initiated Cascade Cyclization of Polyenes with N-terminating Groups

Although cascade cyclization of polyenes has advanced remarkably, most reported examples are limited to carbon- or oxygen-terminating groups, with nitrogen-terminating cyclizations remaining rare. In this work, we developed a cascade cyclization of polyenes with N-terminating groups, initiated by an isolable selenenyl iodide (RSeI) bearing a cavity-shaped substituent. Acid-labile substrates were successfully employed in the cascade cyclization. Compared with commonly used organoselenium reagents, a selenenyl iodide, characterized by its unique soft electrophilic nature, proved to be the most effective in promoting the cascade reaction. Furthermore, the stabilizing effect of the cavity-shaped substituent enabled the isolation of a selenenic acid (RSeOH), which was generated via oxidative β-selenoxide elimination from the cyclized product during its derivatization to an olefin, as a stable compound. We also developed a method for regenerating the starting selenenyl iodide from the selenenic acid. The reusability of the selenenyl iodide renders the series of molecular transformations environmentally benign by reducing unwanted organoselenium waste.

Thyroid Hormone Deiodination-Mechanisms and Small Molecule Enzyme Mimics

Thyroid hormones, biosynthesized in the follicular cells in the thyroid gland, play a crucial role in regulating various important biological processes. The thyroid hormone is synthesized as pro-hormone L-thyroxine (T4), while the active form is primarily produced through the phenolic ring deiodination of T4 by iodothyronine deiodinase enzymes (DIOs). Three distinct isoforms of the enzyme are known, which, despite having almost similar amino acid sequences in their active site, differ in their regioselectivity of deiodination towards T4 and its metabolites. However, the precise mechanism and the origin of the differences in the regioselectivity of deiodination by DIOs are still not fully understood. Over the years, several research groups have attempted to mimic this system with small molecules to gain some insight into the reactivity and mechanism. In this review, we will explore the recent developments on the biomimetic deiodination of T4 and its derivatives by using selenium-based enzyme mimetics. For example, naphthalene-based molecules, featuring a 1,8-dichalcogen atom, have been shown to perform tyrosyl ring deiodination of T4 and T3, producing rT3 and 3,3'-T2, respectively. The modification of the electron density around the phenolic ring through substitutions in the 4'-hydroxyl group can alter the regioselectivity of the deiodination by deiodinase mimics. Additionally, we will highlight the recent progress in the development of a dipeptide-based DIO1 mimic, as well as the deiodination of other halogenated thyronine derivatives by mimics.

Characteristics of patients with Graves' disease who developed drug resistance and required surgery

Although antithyroid drugs (ATDs) are the first-line treatment for patients with Graves' disease (GD) in Japan and other countries, some patients do not achieve remission due to drug resistance, leading to surgery. Even if ATD doses are increased, they often have uncontrolled thyroid function and enlarged goiters, necessitating high-risk emergency surgical treatment. In this study, we aimed to identify the characteristics of patients resistant to ATDs who underwent thyroidectomy and those who achieved remission. We retrospectively analyzed 45 patients with GD who underwent thyroidectomy and 73 patients who achieved remission with ATDs at Shinshu University Hospital between April 1, 2015 and September 30, 2023. In patients who underwent surgery, the drug-resistant patients (DR group; n = 15) had longer disease durations (8.0 vs. 3.0 years, respectively; p = 0.013), higher free triiodothyronine (FT3) / free thyroxine (FT4) ratios (5.54 vs. 3.52, respectively; p = 0.005), higher anti-TSH receptor antibody (TRAb) levels (39.16 vs. 13.31 IU/L, respectively; p = 0.002), and larger thyroid glands (251.00 vs. 54.80 g, respectively; p < 0.001) than non-drug-resistant patients (NDR group; n = 30). Compared with patients who achieved remission with ATDs (Remission group; n = 73), the DR group had higher FT3/FT4 ratios (5.54 vs. 2.99, respectively; p < 0.001) and higher TRAb levels (39.16 vs. 5.9 IU/L, respectively; p < 0.001). Notably, most of the patients in the DR group had a combination of these factors. This suggests that in patients with large thyroid, high FT3/FT4 ratios, and high TRAb levels, early consideration of definitive-curative treatment such as surgery or RI therapy may be warranted instead of continuing prolonged ineffective ATDs treatment.

The role of the hypothalamic-pituitary-thyroid axis in thyroid cancer

The hypothalamic-pituitary-thyroid axis plays a crucial role in the pathogenesis, diagnosis, risk stratification, effectiveness of radioiodine therapy, and treatment response evaluation in epithelial thyroid cancer. Supraphysiological doses of levothyroxine are used in patients with intermediate-risk and high-risk thyroid cancer to suppress thyroid-stimulating hormone (TSH) to prevent tumour progression. However, free thyroxine and tri-iodothyronine have also been found to promote tumour growth in thyroid cancer preclinical models. Moreover, current evidence remains inconclusive about the role of TSH suppression in improving survival outcomes and reveals an increased risk of cardiovascular and skeletal adverse events after long-term exposure to excess levothyroxine. Stimulation of the axis with either recombinant human TSH or thyroid hormone withdrawal has been proven equally effective for diagnostic purposes and for facilitating radioiodine uptake for thyroid remnant ablation, but evidence is insufficient for non-inferiority of recombinant human TSH-based vs thyroid hormone withdrawal-based stimulation before radioiodine therapy of distant metastases.

Effect of Halogen Substitution on the Regioselective Deiodination of Thyroid Hormone Analogues by Deiodinase Mimics

The deiodination of L-thyroxine (T4) by the three isoforms of selenium-containing iodothyronine deiodinases (DIOs) is an important process in the human body for the maintenance of the thyroid hormone homeostasis. The 5'-deiodination of T4 by DIO1 or DIO2 produces the biologically active hormone 3,5,3'-triiodo thyronine (T3) and 5-deiodination by DIO1 or DIO3 generates 3,3',5'-triiodo thyronine (rT3), which is considered to be biologically inactive. Selenium compounds are known to mimic the function of DIO3 by catalyzing the 5-deiodination of T4 and a co-operative halogen and chalcogen bonding is responsible for their activity. The substitution of 4'-OH group of T4 by an electron withdrawing group not only alters the reactivity but also changes the regioselectivity of deiodination. However, the effect of other halogen atoms (F, Cl or Br) in place of iodine in T4 on the regioselectivity has not been explored. In this paper, we describe simple synthetic methodologies toward various halogen analogues of T4. We also show that 2D NMR spectroscopy can be used as a powerful tool to identify the location of various halogen substitutions on the phenolic or tyrosyl rings. The deiodination experiments with peri-substituted naphthalene diselenol reveal that the regioselectivity or specificity towards iodine atom is not altered upon introduction of more electronegative halogen atoms (F, Cl and Br), reinforcing the concept of halogen bonding in the deiodination reactions.

Mechanisms of developmental neurotoxicity mediated by perturbed thyroid hormone homeostasis in the brain: an adverse outcome pathway network

Thyroid hormone (TH) is crucial for proper neurodevelopment. Insufficient TH concentrations in early life are associated with lower IQ and delayed motor development in children. Intracellular levels of TH are modulated via the transmembrane transport of TH and intracellular deiodination, and can mediate gene transcription via binding to the nuclear TH receptor. Chemical exposure can disrupt TH homeostasis via modes of action targeting intracellular mechanisms, thereby potentially influencing TH transport, deiodination or signaling. Understanding the cause and effect relationships of chemical hazards interfering with TH homeostasis in the developing brain is necessary to identify how chemicals might disturb brain development and result in neurodevelopmental disorders. Adverse Outcome Pathways (AOPs) can provide a template for mapping these relationships, and so far multiple AOPs have been developed for TH homeostasis and adverse effects on cognition. The present review aims to expand current AOP networks by (1) summarizing the most important factors in the regulation of brain development under influence of TH, (2) integrating human-based mechanistic information of biological pathways which can be disturbed by TH disrupting chemicals, and (3) by incorporating brain-specific TH-mediated physiology, including barriers and cell specificity, as well as clinical knowledge. TH-specific pathways in the fetal brain are highlighted and supported by distinguishing cell type specific Molecular Initiating Events (MIEs) and downstream Key Events (KEs) for astrocytes, neurons and oligodendrocytes. Two main pathways leading to adverse outcomes (AOs) in the areas of 'cognition' and 'motor function' are decreased myelination due to oligodendrocyte dysfunction, and decreased synaptogenesis and network formation via the neurons. The proposed AOP framework can form a basis for selecting developmental neurotoxic in vitro and in silico test systems for an innovative human-focused hazard testing strategy and risk assessment of chemical exposure.

Rebound effect of hypothalamic-pituitary thyreotropic activity: a new model to better understand hypothyroidism

PURPOSE

Hypothalamic-pituitary thyrotropic activity (HPta) is crucial since TSH is the mainstay for evaluating primary hypothyroidism (hT) and replacement therapy in clinical practice. Despite TSH values, some patients experience symptoms and metabolic alterations, raising several issues about hT. The aim of the study was to investigate factors influencing the TSH values achieved after a period of hT induced in a standardized and controlled manner (TSH_time1).

METHODS

Our institutional database was searched to extract records of differentiated thyroid cancer (DTC) patients undergoing a LT4 withdrawal protocol prior to radioiodine (RAI) administration. We collected clinical and biochemical parameters before LT4 discontinuation and after one month of induced hT. We performed Mann-Whitney U-test and linear regression analyses.

RESULTS

We included 102 patients, with a median age of 44 years. In univariate analyses, TSH_time1 was correlated with age (p 0.005) and the dose pro Kg/die of LT4 assumed until the discontinuation of LT4 (LT4_dose) (p 0.023). The higher the age, the lower the TSH_time1 level. The higher the LT4_dose, the higher the TSH_time1 level. After multivariate analysis, the fittest model included age, BMI, LT4_dose, and systemic inflammation response index with an adjusted R2 of 0.4.

CONCLUSION

The study highlights new mechanisms that influence HPta. HPta progressively reduces with age, and this must be considered when evaluating TSH values in the elderly. Furthermore, we report for the first time a rebound effect of HPta, determined by the dose pro Kg/die of LT4 taken prior to its discontinuation. Inflammation and metabolic status also affect these phenomena.

Selenium metabolism and selenoproteins function in brain and encephalopathy

Selenium (Se) is an essential trace element of the utmost importance to human health. Its deficiency induces various disorders. Se species can be absorbed by organisms and metabolized to hydrogen selenide for the biosynthesis of selenoproteins, selenonucleic acids, or selenosugars. Se in mammals mainly acts as selenoproteins to exert their biological functions. The brain ranks highest in the specific hierarchy of organs to maintain the level of Se and the expression of selenoproteins under the circumstances of Se deficiency. Dyshomeostasis of Se and dysregulation of selenoproteins result in encephalopathy such as Alzheimer's disease, Parkinson's disease, depression, amyotrophic lateral sclerosis, and multiple sclerosis. This review provides a summary and discussion of Se metabolism, selenoprotein function, and their roles in modulating brain diseases based on the most currently published literature. It focuses on how Se is utilized and transported to the brain, how selenoproteins are biosynthesized and function physiologically in the brain, and how selenoproteins are involved in neurodegenerative diseases. At the end of this review, the perspectives and problems are outlined regarding Se and selenoproteins in the regulation of encephalopathy.

Selenium nutritional status and thyroid dysfunction

Selenium(Se) is an essential micronutrient for several immune and regulatory functions in the body. In thyroid tissue, Se contributes to the antioxidant system and is a crucial component of deiodinases, which are selenoproteins that participate in thyroid hormone metabolism. Additionally, this micronutrient exerts a significant impact on thyroid pathophysiology, as low levels of Se lead to reduced activity of glutathione peroxidase, a selenoprotein involved in antioxidative processes, thereby resulting in increased oxidative stress and damage to thyroid tissue. Selenium deficiency (SeD) can cause growth retardation and reproductive failure; in women and children, it may result in Keshan's disease and Kashin-Beck's disease. Research has shown an inverse correlation between Se serum levels and autoimmune thyroiditis in areas with mild SeD. In Graves' disease, Se supplementation has been linked to faster achievement of euthyroidism as well as improvements in quality of life, lessened orbital involvement, and slower ocular progression of the disease. Furthermore, several studies suggest an association between serum SeD and the development of thyroid cancer. Maintaining physiological Se concentrations appears to be related to the prevention of thyroid disease, although current data are insufficient to conclusively support or refute the efficacy of supplementation. Through this narrative review, we aim to present the latest information on the role of selenium in thyroid pathophysiology. To identify relevant literature, specific search strategies were employed in the electronic databases PubMed, Lilacs, and SciELO.

Divergent Photoperiodic Responses in Hypothalamic Dio3 Expression and Gonadal Activity Between Offspring and Paternal Brandt's Voles

The postnatal development of gonadal glands in seasonal breeders, particularly small rodent species, is influenced by photoperiodic patterns. However, little research has been conducted on the effects of pattern similarity and age differentiation especially in molecular features. This study compares the postnatal development of gonadal glands and the expression of hypothalamic genes related to reproductive regulation in male offspring of Brandt's voles (Lasiopodomys brandtii) born under three types of changing photoperiodic patterns: increasing long photoperiod (ILP, 12 h + 3 min/day), natural increasing long photoperiods (NLPs), and decreasing short photoperiods (DSPs, 12 h - 3 min/day), as well as in their paternal voles exposed to these patterns at the same period. Results indicate that over the course of 12 postnatal weeks, gonadal development, including organ masses and serum testosterone levels, exhibited similar profiles between the ILP and NLP groups, which were significantly higher than those observed in DSP offspring. Hypothalamic type 3 iodothyronine deiodinase (Dio3) exhibited significantly higher expression in the DSP group from postnatal week 4 to 8 compared to the other two groups. These physiological and molecular differences gradually decreased with age in offspring, but were never observed in the paternal voles, indicating divergent photoperiodic responses between the two ages. The synchronous profiles observed between hypothalamic Dio3 expression and gonadal activities underscore its crucial role in interpreting photoperiodic signals and regulating gonadal development in Brandt's voles.

The molecular mechanisms of TRβ receptor interaction with polychlorinated biphenyls: A multispectral and computational exploration

The thyroid hormone (TH) system is susceptible to the toxic effects of polychlorinated biphenyls (PCBs). Pollutants may disrupt the TH system by binding to serum TH transport proteins or interacting with thyroid hormone receptors (TRs) in target cells. However, the molecular mechanism of interaction with the Thyroid Hormone Receptor Beta (TRβ) is not fully understood. This study employed fluorescence, UV-visible absorption, three-dimensional fluorescence, and Fourier-transform infrared spectroscopy, along with molecular docking and molecular dynamics simulations, to investigate the interaction between TRβ and PCBs. Moreover, molecular docking and fluorescence resonance energy transfer (FRET) findings suggest that TRβ and PCBs underwent resonance energy transfer consistent with Förster's theory. The root mean square deviation (RMSD) and docking outcomes indicate that the TRβ-PCB29 complex exhibited optimal structural stability. Thus, the study concludes that integrating spectroscopic data with molecular docking is essential for a comprehensive analysis. Further analysis of intermolecular interactions using quantum chemistry and reduced density gradient analysis (RDG) analysis revealed that van der Waals forces are the primary drivers of PCBs to TRβ.

Thyroid Hormone Activation Regulates the Crosstalk between Breast Cancer and Mesenchymal Stem Cells

BACKGROUND

Thyroid Hormones (THs) critically impact human cancer. Although endowed with both tumor-promoting and inhibiting effects in different cancer types, excess of THs has been linked to enhanced tumor growth and progression. Breast cancer depends on the interaction between bulk tumor cells and the surrounding microenvironment in which mesenchymal stem cells (MSCs) exert powerful pro-tumorigenic activities.

METHODS

Primary human MSCs from healthy female donors were co-cultured with DIO2 knock out (D2KO) and wild type (WT) MCF7 breast cancer cells to assess cell growth, migration, invasion and the expression of known epithelial-mesenchymal transition (EMT)- and inflammation-related markers. Furthermore, a surgery-free intraductal delivery model, i.e., the Mouse-INtraDuctal (MIND) injection method, was used as a tool for in vivo characterization of breast tumor formation and progression.

RESULTS

In this study, we uncovered a novel role of THs in regulating the tumor-stroma crosstalk. MCF7 cells enhanced the intracellular activation of THs through the TH-activating enzyme, D2, fostering their EMT properties and the dialogue with MSCs. D2 inactivation reduced the invasiveness of MCF7 cells and their responsiveness to the pro-tumorigenic induction via MSCs, both in vivo and in vitro.

CONCLUSIONS

Thus, we argue that intracellular activation of THs via D2 is a critical requirement for invasive and metastatic conversion of breast cancer cells, advising the blocking of D2 as a potential therapeutic tool for cancer therapy.

Effects of L-NAME and air exposure on mitochondrial energetic markers, thyroid hormone receptor/regulator system and stress/ease-responsive receptor expression in the brain/gut axis of zebrafish

As a signal molecule, nitric oxide (NO) has several physiological actions in fish. However, the action of NO on the brain/gut axis, a classic inter-organal axis that bridges the gastrointestinal tract and the CNS, still requires more understanding. The short-term in vivo action of a NO inhibitor, N-omega-nitro-L-arginine methyl ester (L-NAME), on mitochondrial energetic markers and the receptor expression of thyroid hormone (TH) and neuroendocrine hormones involved in stress/ease response was tested in the brain/gut axis of zebrafish exposed to either in non-stressed or air-exposed condition. L-NAME treatment decreased the NO content in brain and gut segments in non-stressed fish but rose upon L-NAME treatment in air-exposed fish that corresponded with the activation of inos, nnos, hif1a and hif1an transcript expressions. The brain/gut segments that showed spatial and differential sensitivity to L-NAME, modified the transcript expression patterns of stress (adra2da, adrb1, nr3c2)- and ease-responsive (htr2b, slc6a4a, mtnr1aa) hormone receptors. The expression pattern of the TH receptor/regulator system (thra, thrb, dio1, dio2, dio3) becomes more active in gut segments than brain segments upon L-NAME challenge in stressed zebrafish. The data provide evidence for a novel role of NO as an integrator of brain/gut axis segments in zebrafish, where the endogenously produced NO in mid-brain/posterior-gut axis aligns together upon air-exposure stress, providing a lead role to the posterior gut that activates and directs the neuroendocrine receptor expressions of stress/ease responsive genes. The data further invites studies exploring the therapeutic potential of L-NAME in this biomedical model to control the brain/gut axis segments.

Thyroid hormone and the Liver

It is known that thyroid hormone can regulate hepatic metabolic pathways including cholesterol, de novo lipogenesis, fatty acid oxidation, lipophagy, and carbohydrate metabolism. Thyroid hormone action is mediated by the thyroid hormone receptor (THR) isoforms and their coregulators, and THRβ is the main isoform expressed in the liver. Dysregulation of thyroid hormone levels, as seen in hypothyroidism, has been associated with dyslipidemia and metabolic dysfunction-associated fatty liver disease. Given the beneficial effects of thyroid hormone in liver metabolism and the advances illuminating the use of thyroid hormone analogs such as resmetirom as therapeutic agents in the treatment of metabolic dysfunction-associated fatty liver disease, this review aims to further explore the relationship between TH, the liver, and metabolic dysfunction-associated fatty liver disease. Herein, we summarize the current clinical therapies and highlight future areas of research.

Tanycytes from a bird's eye view: gene expression profiling of the tanycytic region under different seasonal states in the Svalbard ptarmigan

In mammals and birds, tanycytes are known to regulate thyroid hormone conversion, and this process is central to the control of seasonal reproduction. In mammals, this cell type is also implicated in retinoic acid signalling, neurogenesis, and nutritional gatekeeping, all of which have been linked to hypothalamic regulation of energy metabolism. Less is known about these potential wider roles of tanycytes in birds. To address this gap, we combined LASER capture microdissection and transcriptomics to profile the tanycytic region in male Svalbard ptarmigan, a High Arctic species with photoperiod-dependent seasonal rhythms in reproductive activation and body mass. Short photoperiod (SP) adapted birds were transferred to constant light (LL) to trigger breeding and body mass loss. After five months under LL, the development of photorefractoriness led to spontaneous re-emergence of the winter phenotype, marked by the termination of breeding and gain in body mass. The transfer from SP to LL initiated gene expression changes in both thyroid hormone and retinoic acid pathways, as described in seasonal mammals. Furthermore, transcriptomic signatures of cell differentiation and migration were observed. Comparison to data from Siberian hamsters demonstrated that a photoperiod-dependent re-organisation of the hypothalamic tanycytic region is likely a conserved feature. Conversely, the spontaneous development of photorefractoriness showed a surprisingly small number of genes that reverted in expression level, despite reversal of the reproductive and metabolic phenotype. Our data suggest general conservation of tanycyte biology between photoperiodic birds and mammals and raise questions about the mechanistic origins of the photorefractory state.

Regulation mechanism of endochondral ossification in Rana zhenhaiensis during metamorphosis based on histomorphology and transcriptome analyses

Endochondral ossification plays a crucial role in the limb development of amphibians. This study explored the ossification sequence in the hindlimb of Rana zhenhaiensis tadpoles and the correlation between thyroid hormones (THs) and endochondral ossification via histomorphology and transcriptional analyses. Our results suggest that ossification of the femur and tibiofibula was initiated during the period of high THs activity (metamorphosis climax). In addition, the results of differentially expressed gene analyses in the hindlimb and tail showed that systemic factors, transcription factors, and locally secreted factors interacted with each other during the metamorphosis climax to regulate the occurrence of endochondral ossification. These results will enrich the morphological data of anurans and provide scientific reference for the evolutionary history of vertebrates.

PKCα Activation via the Thyroid Hormone Membrane Receptor Is Key to Thyroid Cancer Growth

Thyroid carcinoma (TC) is the most common endocrine neoplasia, with its incidence increasing in the last 40 years worldwide. The determination of genetic and/or protein markers for thyroid carcinoma could increase diagnostic precision. Accumulated evidence shows that Protein kinase C alpha (PKCα) contributes to tumorigenesis and therapy resistance in cancer. However, the role of PKCα in TC remains poorly studied. Our group and others have demonstrated that PKCs can mediate the proliferative effects of thyroid hormones (THs) through their membrane receptor, the integrin αvβ3, in several cancer types. We found that PKCα is overexpressed in TC cell lines, and it also appeared as the predominant expressed isoform in public databases of TC patients. PKCα-depleted cells significantly reduced THs-induced proliferation, mediated by the integrin αvβ3 receptor, through AKT and Erk activation. In databases of TC patients, higher PKCα expression was associated with lower overall survival. Further analyses showed a positive correlation between PKCα and genes from the MAPK and PI3K-Akt pathways. Finally, immunohistochemical analysis showed abnormal upregulation of PKCα in human thyroid tumors. Our findings establish a potential role for PKCα in the control of hormone-induced proliferation that can be explored as a therapeutic and/or diagnostic target for TC.

Structural Insights into the Iodothyronine Deiodinase 2 Catalytic Core and Deiodinase Catalysis and Dimerization

Iodothyronine deiodinases (Dio) are selenocysteine-containing membrane enzymes that activate and inactivate the thyroid hormones (TH) through reductive iodide eliminations. The three deiodinase isoforms are homodimers sharing highly conserved amino acid sequences, but they differ in their regioselectivities for the deiodination reaction and regulatory features. We have now solved a crystal structure of the mouse deiodinase 2 (Dio2) catalytic domain. It reveals a high overall similarity to the deiodinase 3 structure, supporting the proposed common mechanism, but also Dio2-specific features, likely mediating its unique properties. Activity studies with an artificially enforced Dio dimer further confirm that dimerization is required for activity and requires both the catalytic core and the enzyme's N-terminus. Cross-linking studies reveal the catalytic core's dimerization interface, providing insights into the architecture of the complete, active Dio homodimer.

The Downregulation of the Liver Lipid Metabolism Induced by Hypothyroidism in Male Mice: Metabolic Flexibility Favors Compensatory Mechanisms in White Adipose Tissue

In mammals, the maintenance of energy homeostasis relies on complex mechanisms requiring tight synchronization between peripheral organs and the brain. Thyroid hormones (THs), through their pleiotropic actions, play a central role in these regulations. Hypothyroidism, which is characterized by low circulating TH levels, slows down the metabolism, which leads to a reduction in energy expenditure as well as in lipid and glucose metabolism. The objective of this study was to evaluate whether the metabolic deregulations induced by hypothyroidism could be avoided through regulatory mechanisms involved in metabolic flexibility. To this end, the response to induced hypothyroidism was compared in males from two mouse strains, the wild-derived WSB/EiJ mouse strain characterized by a diet-induced obesity (DIO) resistance due to its high metabolic flexibility phenotype and C57BL/6J mice, which are prone to DIO. The results show that propylthiouracil (PTU)-induced hypothyroidism led to metabolic deregulations, particularly a reduction in hepatic lipid synthesis in both strains. Furthermore, in contrast to the C57BL/6J mice, the WSB/EiJ mice were resistant to the metabolic dysregulations induced by hypothyroidism, mainly through enhanced lipid metabolism in their adipose tissue. Indeed, WSB/EiJ mice compensated for the decrease in hepatic lipid synthesis by mobilizing lipid reserves from white adipose tissue. Gene expression analysis revealed that hypothyroidism stimulated the hypothalamic orexigenic circuit in both strains, but there was unchanged melanocortin 4 receptor (Mc4r) and leptin receptor (LepR) expression in the hypothyroid WSB/EiJ mice strain, which reflects their adaptability to maintain their body weight, in contrast to C57BL/6J mice. Thus, this study showed that WSB/EiJ male mice displayed a resistance to the metabolic dysregulations induced by hypothyroidism through compensatory mechanisms. This highlights the importance of metabolic flexibility in the ability to adapt to disturbed circulating TH levels.

Cross-Sectional Examination of Thyroid Hormones and Environmental Exposure to Multiclass Pesticides in Women of Reproductive Age in China

BACKGROUND

Some pesticides have been shown to interfere with thyroid functions through changes in thyroid hormone (TH) levels. However, few human studies have explored associations between TH levels and environmental exposure to currently used pesticides, including neonicotinoids, phenylpyrazoles, phenoxy acids, and azoles. Moreover, such studies often measure biomarkers of exposure in urine or blood, and thus reveal only recent exposure. In contrast, hair has been demonstrated to be a suitable matrix for assessing chronic exposure to both persistent and nonpersistent organic pollutants.

OBJECTIVES

We investigated 54 biomarkers of pollutant exposure in relation to tetraiodothyronine (T4), 3,3',5-triiodothyronine (T3), 3,3',5'-triiodothyronine (rT3), and 3,3'-diiodothyronine (T2).

METHODS

In a cross-sectional study of 196 healthy Chinese women of reproductive age (25-45 years of age), concentrations of both pollutants and THs were analyzed in the first 12 cm (starting from the scalp) of the hair matrix, collected in 2016. Associations between pollutants and TH levels were explored using stability-enhanced least absolute shrinkage and selection operator (lasso) by regressing all exposures against each outcome of interest, adjusted for age, body mass index, and city.

RESULTS

Each TH was associated with the mixture of at least eight of the examined pesticides. We found associations of β -HCH, PCP, DMP, DETP, 3Me4NP, carbofuran, ClCF 3 CA , imidacloprid, 2,4-D, metolachlor, difenoconazole, and tebuconazole with THs. For example, a 2-standard deviation (SD) increase in log 10 -transformed hair DMP concentration was associated with lower hair T4 concentration [- 15.0 % (95% CI: - 26.1 , - 2.21 % )] and higher hair T3 concentration [8.16% (95% CI: 1.73, 15.0%)] in the adjusted unpenalized regression models. We also found associations of some pesticides with T3/T4, rT3/T4, and rT3/T3 molar ratios, including PCP, DMP, 2,4-D, metolachlor, difenoconazole, and tebuconazole.

DISCUSSION

Our results suggest that exposure to the low levels of pesticides examined here may disrupt thyroid homeostasis in humans. Further studies are needed to confirm our results and to evaluate the long-term consequences of these subtle interferences. https://doi.org/10.1289/EHP14378.

Early-life exposure to five biodegradable plastics impairs eye development and visually-mediated behavior through disturbing hypothalamus-pituitary-thyroid (HPT) axis in zebrafish larvae

Biodegradable plastics have been commonly developed and applied as an alternative to traditional plastics, which cause environmental plastic pollution. However, biodegradable plastics still present limitations such as stringent degradation conditions and slow degradation rate, and may cause harm to the environment and organisms. Consequently, in this study, zebrafish was used to evaluate the effects of five biodegradable microplastics (MPs), polyglycolic acid (PGA), polylactic acid (PLA), polybutylene succinate (PBS), polyhydroxyalkanoate (PHA) and polybutylene adipate terephthalate (PBAT) exposure on the early development, retina morphology, visually-mediated behavior, and thyroid signaling at concentrations of 1 mg/L and 100 mg/L. The results indicated that all MPs induced decreased survival rate, reduced body length, smaller eyes, and smaller heads, affecting the early development of zebrafish larvae. Moreover, the thickness of retinal layers, including inner plexiform layer (IPL), outer nuclear layer (ONL), and retinal ganglion layer (RGL) was decreased, and the expression of key genes related to eye and retinal development was abnormally altered after all MPs exposure. Exposure to PBS and PBAT led to abnormal visually-mediated behavior, indicating likely affected the visual function. All MPs could also cause thyroid system disorders, among which alterations in the thyroid hormone receptors (TRs) genes could affect the retinal development of zebrafish larvae. In summary, biodegradable MPs exhibited eye developmental toxicity and likely impaired the visual function in zebrafish larvae. This provided new evidence for revealing the effects of biodegradable plastics on aquatic organism development and environmental risks to aquatic ecosystems.

The thyroid hormone system disrupting potential of resorcinol in fish

Environmental pollutants capable of interfering with the thyroid hormone (TH) system increasingly raise concern for both human and environmental health. Recently, resorcinol has received attention as a compound of concern due to its endocrine disrupting properties. It is a known inhibitor of thyroperoxidase (TPO), an enzyme required in TH synthesis, and therapeutic use of resorcinol exposure has led to hypothyroidism in humans. There is limited evidence concerning ecotoxicologically relevant effects of resorcinol in fish. A set of adverse outcome pathways (AOPs) has recently been developed linking thyroid hormone system disruption (THSD) to impaired swim bladder inflation and eye development in fish. In the present study, these AOPs were used to provide the background for testing potential THSD effects of resorcinol in zebrafish eleutheroembryos. We exposed zebrafish eleutheroembryos to resorcinol and assessed TH levels, swim bladder inflation and eye morphology. As a TPO inhibitor, resorcinol is expected to affect TH levels and eye morphology but not swim bladder inflation during embryonic development. Indeed, thyroxine (T4) levels were significantly decreased following resorcinol exposure. In contrast to our hypothesis, swim bladder inflation was impaired at 5 days post fertilization (dpf) and no effects on eye morphology were detected. Therefore, in vitro assays were performed to identify potential additional thyroid hormone system disruption-related mechanisms through which resorcinol may act. Two new mechanisms were identified: TH receptor (TR) antagonism and transthyretin (TTR) binding inhibition. Both of these mechanisms can plausibly be linked to impaired swim bladder inflation and could, therefore, explain the observed effect. Overall, our study contributes to the knowledge of the THSD potential of resorcinol both in vivo in the zebrafish model as well as in vitro.

Analysis of potential genes, immunological and antioxidant profiles associated with trypanosomiasis susceptibility in dromedary camels

Trypanosomiasis is associated with tissue damage and may trigger an immunological response. These tissue lesions are linked to metabolic issues and oxidative stress. The current study aimed to investigate the immunological, antioxidant, and metabolic changes that may be connected to camel trypanosomiasis. Blood samples were collected from 54 camels and allocated into two groups: The control group (35 camels) and the infected group (19 camels). The genes TLR2, TLR5, IL-17, MARCHF3, RASGRP1, EPS15L1, PPIE, ASB16, CMPK2, LPCAT1, FPGT, GPHN, TNNI3K, DIO3, keap1, and OXSR1 were significantly up-regulated in trypanosomiasis camels. However, down-regulation was observed for the genes Nrf2, PRDX6, and NDUFS5. PCR-DNA sequencing was used to identify nucleotide sequence polymorphisms in the immune (TLR2, TLR5, IL-17, MARCHF3, RASGRP1, and EPS15L1), metabolic (PPIE, ASB16, CMPK2, LPCAT1, FPGT, GPHN, TNNI3K, and DIO3), and antioxidant (Nrf2, Keap1, PRDX6, NDUFS5, and OXSR1) genes between healthy and trypanosomiasis-affected camels. Exploring the serum profile also showed a significant (P ˂ 0.05) increase in Hp, SAA, Cp, IL-1β, IL-6, IL 10, TNF-α, and MDA, with significant (P ˂ 0.05) reduction in the serum levels of CAT, SOD, GSH, T3, and T4 in diseased camels compared with healthy ones. Our findings confirm the significance of nucleotide variations, gene expression patterns, and the biochemical profile of the investigated markers as indicators for the susceptibility of trypanosomiasis in dromedary camels and may be utilized to create management strategies.

Changes in Central Sensitivity to Thyroid Hormones vs. Urine Iodine during Pregnancy

INTRODUCTION/AIM

Central sensitivity to thyroid hormones refers to the responsiveness of the hypothalamic-pituitary-thyroid (HPT) axis to changes in circulating free thyroxine (fT4). Although dose-response relationships between thyroid hormones per se and urinary iodine (UI) levels have been observed, central sensitivity to thyroid hormones in relation to UI remains unexplored. The aim of the present study was to evaluate central sensitivity to thyroid hormones (by means of the Thyroid Feedback Quantile-based Index [TFQI], which is a calculated measure, based on TSH and fT4, that estimates central sensitivity to thyroid hormones) in pregnancy and to assess whether it differs according to gestational age and/or iodine intake.

MATERIALS AND METHODS

One thousand, one hundred and two blood and urine samples were collected from pregnant women (with a mean age ± SD of 30.4 ± 4.6 years) during singleton pregnancies; women with known/diagnosed thyroid disease were excluded. Specifically, TSH and fT4, anti-thyroid peroxidase antibodies and UI were measured in each trimester and at two months postpartum, while the TFQI was calculated for all the study samples. After the elimination of outliers, statistical analysis was conducted with analysis of variance (ANOVA) for the variables versus time period, while Pearson's correlation was used to assess the TFQI versus UI.

RESULTS

The mean TFQI index ranged from -0.060 (second trimester) to -0.053 (two months postpartum), while the corresponding UI was 137 and 165 μg/L, respectively. The TFQI-UI correlation was marginally negative (Pearson r: -0.323, p: 0.04) and significantly positive (r: +0.368, p: 0.050) for UI values over 250 μg/L, in the first and the second trimesters of pregnancy, respectively.

DISCUSSION

The TFQI is a new index reflecting central sensitivity to thyroid hormones. A lower TFQI indicates higher sensitivity to thyroid hormones. In our sample, the TFQI was mainly positively related to iodine intake in the second trimester of pregnancy (following the critical period of organogenesis). Thus, the observed changes in the TFQI may reflect the different ways of the central action of thyroid hormones, according to the phase of pregnancy. These results have the potential to enhance our comprehension of the changes in the HPT axis' function via variations in central sensitivity to thyroid hormones and its interplay with nutritional iodine status during pregnancy.

The growing complexity of the control of the hypothalamic pituitary thyroid axis and brown adipose tissue by leptin

The balance between food intake and energy expenditure is precisely regulated to maintain adipose stores. Leptin, which is produced in and released from adipose in direct proportion to its size, is a major contributor to this control and initiates its homeostatic responses largely via binding to leptin receptors (LepR) in the hypothalamus. Decreases in hypothalamic LepR binding signals starvation, leading to hunger and reduced energy expenditure, whereas increases in hypothalamic LepR binding can suppress food intake and increase energy expenditure. However, large gaps persist in the specific hypothalamic sites and detailed mechanisms by which leptin increases energy expenditure, via the parallel activation of the hypothalamic pituitary thyroid (HPT) axis and brown adipose tissue (BAT). The purpose of this review is to develop a framework for the complex mechanisms and neurocircuitry. The core circuitry begins with leptin binding to receptors in the arcuate nucleus, which then sends projections to the paraventricular nucleus (to regulate the HPT axis) and the dorsomedial hypothalamus (to regulate BAT). We build on this core by layering complexities, including the intricate and unsettled regulation of arcuate proopiomelanocortin neurons by leptin and the changes that occur as the regulation of the HPT axis and BAT is engaged or modified by challenges such as starvation, hypothermia, obesity, and pregnancy.

Phthalate exposure during pregnancy and its association with thyroid hormones: A prospective cohort study

Phthalate esters (PAEs) possess endocrine-disrupting properties. Studies in humans have indicated that in utero phthalate exposure affects maternal thyroid hormones, which are essential for fetal growth and development. However, these studies also reported inconsistent results on the relationship between phthalates and thyroid hormones. This prospective cohort study aimed to assess phthalate exposure across the three trimesters of pregnancy and its association with thyroid hormone levels. From 2019 to 2022, we recruited 672 pregnant women, and two urine samples and one blood sample were collected from each participant during the pregnancy. We examined the urine samples from 663, 335, and 294 women in the first, second, and third trimester, respectively, for the following seven phthalate metabolites: monoethyl phthalate (MEP) from diethyl phthalate (DEP); mono-n-butyl phthalate (MnBP) and mono-iso-butyl phthalate (MiBP) from dibutyl phthalate (DBP); monobenzyl phthalate (MBzP) from butyl benzyl phthalate; and three di(2-ethylhexyl) phthalate (DEHP) metabolites, mono-(2-ethyl-5-hydroxyhexyl) phthalate (MEHHP), mono-(2-ethyl-5-oxohexyl) phthalate (MEOHP), and mono-(2-ethyl-5-carboxypentyl) phthalate (MECPP). Additionally, we examined the levels of free thyroxine (FT4), thyroid-stimulating hormone (TSH), and total triiodothyronine (TT3) in the serum samples of the following participants: 596, 627, and 576 in the first trimester; 292, 293, and 282 in the second trimester; and 250, 250, and 248 in the third trimester, respectively. Other than MBzP, which was detected in 25%-33% of the samples, other metabolites were detectable in >86% of urine samples, indicating widespread exposure to DEP, DBP, and DEHP. The detected phthalate exposure levels in our cohort were significantly higher than those reported in other countries. Metabolite levels varied across the trimesters, implying changes in exposure and metabolism throughout pregnancy. The observed variability in urinary concentrations of phthalate metabolites, which ranged from poor to moderate, underscores the importance of taking multiple measurements during pregnancy for precise exposure assessment. Using a linear mixed model, we analyzed the effects of repeated phthalate exposure on thyroid hormone levels while adjusting for potential confounders. We observed significant linear trends in FT4, TSH, and, to a lesser extent, TT3 across quartiles of specific phthalate metabolites. Comparing the highest to the lowest quartiles, we found a significant increase in FT4 levels, ranging from 2 to 3.7%, associated with MEP; MECPP; MEHHP; and the sum of seven metabolites (∑7PAE), three DEHP metabolites (∑3DEHP), two DBP metabolites (∑DBP), and both low molecular weight (∑LMW) and high molecular weight metabolites. Increased TSH levels (5%-16%) were observed for all phthalate metabolites (except MEHHP) and their molar sums, including ∑7PAE. For TT3, a significant increase was observed with MEP (2.2%) and a decrease was observed with ∑DBP (-2.7%). A higher TSH/FT4 ratio was observed with the highest quartiles (third or fourth) of several phthalate metabolites: MEP (8.8%), MiBP (8.7%), MnBP (22.2%), ∑7PAE (15.3%), ∑DBP (16.4%), and ∑LMW (18.6%). These hormonal alterations, most notably in the second and third trimesters, suggest that phthalate exposure may impact fetal growth and development by affecting maternal thyroid function.

A DIO2 missense mutation and its impact on fetal response to PRRSV infection

BACKGROUND

Porcine reproductive and respiratory syndrome virus 2 (PRRSV-2) infection during late gestation substantially lowers fetal viability and survival. In a previous genome-wide association study, a single nucleotide polymorphism on chromosome 7 was significantly associated with probability of fetuses being viable in response to maternal PRRSV-2 infection at 21 days post maternal inoculation. The iodothyronine deiodinase 2 (DIO2) gene, located ~ 14 Kilobase downstream of this SNP, was selected as a priority candidate related to fetal susceptibility following maternal PRRSV-2 infection. Our objectives were to identify mutation(s) within the porcine DIO2 gene and to determine if they were associated with fetal outcomes after PRRSV-2 challenge. Sequencing of the DIO2, genotyping identified variants, and association of DIO2 genotypes with fetal phenotypes including DIO2 mRNA levels, viability, survival, viral loads, cortisol and thyroid hormone levels, and growth measurements were conducted.

RESULTS

A missense variant (p.Asn91Ser) was identified in the parental populations from two independent PRRSV-2 challenge trials. This variant was further genotyped to determine association with fetal PRRS outcomes. DIO2 mRNA levels in fetal heart and kidney differed by the genotypes of Asn91Ser substitution with significantly greater DIO2 mRNA expression in heterozygotes compared with wild-type homozygotes (P < 0.001 for heart, P = 0.002 for kidney). While Asn91Ser did not significantly alter fetal viability and growth measurements, interaction effects of the variant with fetal sex or trial were identified for fetal viability or crown rump length, respectively. However, this mutation was not related to dysregulation of the hypothalamic-pituitary-adrenal and thyroid axis, indicated by no differences in circulating cortisol, T4, and T3 levels in fetuses of the opposing genotypes following PRRSV-2 infection.

CONCLUSIONS

The present study suggests that a complex relationship among DIO2 genotype, DIO2 expression, fetal sex, and fetal viability may exist during the course of fetal PRRSV infection. Our study also proposes the increase in cortisol levels, indicative of fetal stress response, may lead to fetal complications, such as fetal compromise, fetal death, or premature farrowing, during PRRSV infection.

Are resting metabolic rate and clinical symptoms affected by variation of serum thyroid stimulating hormone levels within the normal range in healthy and women with hypothyroidism? A case-control study

BACKGROUND

It is unclear whether variation in thyroid stimulating hormone (TSH) levels within the reference range affect energy expenditure and clinical symptoms and even within the normal range of TSH levels, resting energy expenditure may alter. The aim of the present study was to determine whether treated hypothyroid subjects and healthy subjects with a low-normal TSH range (0.3-2.3 mIU/L) have better clinical outcomes and increased energy expenditure than those with a high-normal TSH range (2.3-4.3 mIU/L).

METHODS

This was a case-control study of 160 overweight/obese women with TSH levels across the reference range of 0.3-4.3 mU/l. Subjects were paired in four groups: healthy subjects with low-normal target TSH (n = 40), healthy subjects with high-normal target TSH (n = 40), subjects with treated hypothyroidism with low-normal target TSH (n = 40), and subjects with treated hypothyroidism with high-normal target TSH (n = 40). Resting energy expenditure (RMR), dietary intake, body composition, physical activity, and biochemical markers were assessed.

RESULTS

Subjects with low-normal (≤2.3 mU/L) and high-normal (>2.3 mU/L) TSH levels did not differ in terms of RMR, serum T3 levels, and clinical symptoms except fatigue (P = 0.013). However, serum fT4 levels were found to be significantly different between the study groups (P = 0.002). Serum fT4 concentration was the highest in subjects with treated hypothyroidism with low-normal target TSH.

CONCLUSION

Variation in serum TSH levels within the reference range did not significantly affect REE and clinical symptoms except fatigue in healthy and women with hypothyroidism.

Evaluation of Health-Related Quality of Life in Patients with Euthyroid Hashimoto's Thyroiditis under Long-Term Levothyroxine Therapy: A Prospective Case-Control Study

Objectives: To investigate quality of life using the SF-12 scale in euthyroid Hashimoto's thyroiditis patients on levothyroxine therapy for at least three years. Methods: This prospective case-control study included 44 euthyroid Hashimoto's thyroiditis patients and 44 matched controls, conducted at a university hospital's endocrinology clinic from 6 November to 30 December 2023. Participants completed the SF-12 questionnaire; data were analyzed using Shapiro-Wilk, Student's t-test, Mann-Whitney U, Yates chi-squared, and Spearman's tests. Results: The study involved 88 participants (Hashimoto's group: 35 females, 9 males; control group: 31 females, 13 males), with average ages of 49.50 and 47.43 years old, respectively. Significant differences were observed in TSH, T4 levels, and family history (p < 0.05). The Hashimoto's thyroiditis group showed higher thyroid peroxidase antibodies (95.69 IU/mL) and lower scores on both physical and mental sub-dimensions of SF-12, with a significant difference in physical scores (p < 0.05). Significant correlations were found between age and Anti-TG; Anti-TPO and Anti-TG; BMI and T3; TSH and T4; HDL and triglycerides; MCS-12 and PCS-12; Anti-TPO and T3; cholesterol and T3; and LDL and cholesterol (p < 0.05). Other variables showed no significant correlations (p > 0.05). Conclusions: Our study shows that effective control of hypothyroidism is not sufficient to reduce the negative effects of Hashimoto's thyroiditis on patients' health-related quality of life. Beyond the normalization of hormone levels, comprehensive therapeutic strategies targeting the autoimmune aspects of the disease are essential for the management of Hashimoto's thyroiditis. This study provides a foundation for developing effective therapies that can enhance quality of life for patients with Hashimoto's thyroiditis.

Evaluation of Health-Related Quality of Life in Patients with Euthyroid Hashimoto’s Thyroiditis under Long-Term Levothyroxine Therapy: A Prospective Case-Control Study

Objectives: To investigate quality of life using the SF-12 scale in euthyroid Hashimoto’s thyroiditis patients on levothyroxine therapy for at least three years. Methods: This prospective case–control study included 44 euthyroid Hashimoto’s thyroiditis patients and 44 matched controls, conducted at a university hospital’s endocrinology clinic from 6 November to 30 December 2023. Participants completed the SF-12 questionnaire; data were analyzed using Shapiro–Wilk, Student’s t-test, Mann–Whitney U, Yates chi-squared, and Spearman’s tests. Results: The study involved 88 participants (Hashimoto’s group: 35 females, 9 males; control group: 31 females, 13 males), with average ages of 49.50 and 47.43 years old, respectively. Significant differences were observed in TSH, T4 levels, and family history (p < 0.05). The Hashimoto’s thyroiditis group showed higher thyroid peroxidase antibodies (95.69 IU/mL) and lower scores on both physical and mental sub-dimensions of SF-12, with a significant difference in physical scores (p < 0.05). Significant correlations were found between age and Anti-TG; Anti-TPO and Anti-TG; BMI and T3; TSH and T4; HDL and triglycerides; MCS-12 and PCS-12; Anti-TPO and T3; cholesterol and T3; and LDL and cholesterol (p < 0.05). Other variables showed no significant correlations (p > 0.05). Conclusions: Our study shows that effective control of hypothyroidism is not sufficient to reduce the negative effects of Hashimoto’s thyroiditis on patients’ health-related quality of life. Beyond the normalization of hormone levels, comprehensive therapeutic strategies targeting the autoimmune aspects of the disease are essential for the management of Hashimoto’s thyroiditis. This study provides a foundation for developing effective therapies that can enhance quality of life for patients with Hashimoto’s thyroiditis.

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.

Factors and Mechanisms of Thyroid Hormone Activity in the Brain: Possible Role in Recovery and Protection

Thyroid hormones (THs) are essential in normal brain development, and cognitive and emotional functions. THs act through a cascade of events including uptake by the target cells by specific cell membrane transporters, activation or inactivation by deiodinase enzymes, and interaction with nuclear thyroid hormone receptors. Several thyroid responsive genes have been described in the developing and in the adult brain and many studies have demonstrated a systemic or local reduction in TH availability in neurologic disease and after brain injury. In this review, the main factors and mechanisms associated with the THs in the normal and damaged brain will be evaluated in different regions and cellular contexts. Furthermore, the most common animal models used to study the role of THs in brain damage and cognitive impairment will be described and the use of THs as a potential recovery strategy from neuropathological conditions will be evaluated. Finally, particular attention will be given to the link observed between TH alterations and increased risk of Alzheimer's Disease (AD), the most prevalent neurodegenerative and dementing condition worldwide.

Elevation of free triiodothyronine (fT3) levels by Plasmodium falciparum independent of thyroid stimulating hormone (TSH) in children with uncomplicated malaria

BACKGROUND

Malaria parasites have a devastating effect on the infected host. However, there is a paucity of data on the effect of Plasmodium falciparum on thyroid hormones.

METHODS

This case-control study (1:1) involved children <16 years of age with uncomplicated malaria. Hematological parameters were determined using the URIT-5380 hematology analyzer (China). Later, levels of thyroid hormones, namely free triiodothyronine (fT3), free tetraiodothyronine (fT4), and thyroid-stimulating hormone (TSH), were determined using human ELISA kits (DiaSino ELISA kit, Zhengzhou, China).

RESULTS

Ninety children with malaria and ninety matched control group were studied. Overall, compared to the control group, lower TSH (3.43 ± 1.25 vs. 3.84 ± 1.34, p = 0.035) and elevated levels of fT3 levels (5.85 ± 1.79 vs. 3.89 ± 1.19, p < 0.001) were observed in patients with malaria. However, fT4 levels were comparable between cases and control group (16.37 ± 2.81 vs 17.06 ± 3.5, p = 0.150). Free T3 levels were significantly higher in children <10 years (p < 0.001) and higher among male children with malaria (p < 0.001). Overall, there was a significant positive relationship between parasite counts and fT3 (R = 0.95, p < 0.001). Furthermore, body temperature was positively correlated with fT3 (R = 0.97, p < 0.001).

CONCLUSIONS

Isolated fT3 thyrotoxicosis was observed in falciparum malaria, especially in children <10 years and male malaria patients, independent of TSH. This observation could explain the severity of malaria in children.

A combination of silymarin and garlic extract enhances thyroid hormone activation and body metabolism in orally intoxicated male rats with atrazine: Impact on hepatic iodothyronine deiodinase type 1

Atrazine is a widely applied herbicide to improve crop yield and maintain general health. It has been reported to impair thyroid function and architecture in experimental animals. Alterations in thyroid hormones disrupt normal body function and metabolism. Silymarin, a hepatoprotective flavonolignan, was found to improve thyroid function and body metabolism. Additionally, garlic displays several protective effects on body organs. Therefore, this study explored the prophylactic impact of natural compounds comprising silymarin and garlic extract on disrupted thyroid function, hepatic iodothyronine deiodinase type 1, and metabolic parameters in atrazine-intoxicated male rats. We found that daily pre- and co-treatment of atrazine-intoxicated male rats with silymarin (100 mg/kg, p.o) and/or garlic extract (10 mg/kg, p.o) significantly improved thyroid activation and hepatic functionality as evidenced by the re-establishment of T3, T3/T4, and TSH values as well as ALT and AST activities. Interestingly, individual or concurrent supplementation of the atrazine group with silymarin and garlic extract prevented the down-regulation in hepatic iodothyronine deiodinase type 1. These effects were coupled with the repletion of serum and hepatic antioxidants and the amelioration of lipid peroxidation. In addition, current natural products markedly alleviated weight gain, dyslipidemia, hyperglycemia, glucose intolerance, and insulin resistance. Notably, a cocktail of silymarin and garlic extract exerted superior protection against atrazine-triggered deterioration of thyroid, hepatic, and metabolic functioning to individual treatments. Present findings pinpoint the prophylactic and synergistic influence of silymarin and garlic extract combinatorial regimen on thyroid activation and body metabolism via enhancing antioxidant potential, maintaining hepatic function, and iodothyronine deiodinase type 1.

A Novel and Fast Online-SPE-LC-MS/MS Method to Quantify Thyroid Hormone Metabolites in Rat Plasma

Since the focus in regulatory toxicology has drifted toward the identification of endocrine disruptors, the improvement in determination of alterations in the thyroid hormone system has become more important. THs are involved in several molecular processes important for a proper pre- and postnatal development so that disturbances can inter alia lead to incorrect brain maturation and/or disturbed metabolic processes (thermogenesis or lipolysis). In this publication, a new automated online solid-phase extraction (SPE)-liquid chromatography (LC)-tandem mass spectrometry (MS/MS, xLC-MS/MS) is introduced which simultaneously analyzes total T4, T3, rT3, T2, and T1. Method validation parameters are presented, and the method was positively verified by analyzing control and PTU-treated rat plasma samples (time points day 7, 14, and 28) for their total TH content. The obtained results were compared to published results by using a radioimmunoassay method. The automated SPE system ensures a consistent unified sample preparation, and this method overall showed sufficient specificity and accuracy to detect the given analytes in rat plasma. For the preparation of 50 μL of rat plasma, the following LOQs were established: 0.020 nM for T1, 0.029 nM for T2, 0.023 nM for rT3 and T3, and 3.22 nM for T4. This method is suitable to assess the identification of mechanisms leading to adverse effects, such as disturbed TH metabolism and regulation.

Structure-Related Thyroid Disrupting Effect of Perfluorooctanesulfonate-like Substances in Zebrafish Larvae

Chlorinated polyfluorooctane ether sulfonate (6:2 Cl-PFESA), hydrogenated polyfluorooctane ether sulfonate (6:2 H-PFESA), and chlorinated polyfluorooctanesulfonate (Cl-PFOS) share structural similarities with the regulated perfluorooctanesulfonate (PFOS), but their toxic potential is rarely known. Here, the thyroid disrupting potential of these four compounds in zebrafish larvae has been comparably investigated. PFOS, Cl-PFOS, and 6:2 Cl-PFESA were accumulated in the larvae at similar levels, approximately 1.3-1.6 times higher than 6:2 H-PFESA. Additionally, PFOS, Cl-PFOS, and 6:2 Cl-PFESA exhibited stronger disruption than 6:2 H-PFESA on genetic regulation, particularly concerning thyroid hormone (TH) activation and action and on TH homeostasis in both free and total forms of thyroxine (T4) and 3,5,3'-triiodothyronine (T3). These results indicate that chlorination or oxygen insertion does not substantially alter the thyrotoxicity of PFOS, but hydrogenation mitigates it. Molecular docking analysis and the luciferase reporter gene assay provided mechanistic perspectives that the PFOS-like substances could competitively replace THs to bind with TH plasma and membrane transporters, thereby disrupting TH transport and action, respectively. Moreover, they are also potent to disrupt TH synthesis and activation through Na+/K+-dependent transport of I- or competitive binding to the sites of deiodinases.

Local modulation of thyroid hormone signaling in the retina affects the development of diabetic retinopathy

Thyroid hormone (TH) dyshomeostasis is associated with poor prognosis in acute and prolonged illness, but its role in diabetic retinopathy (DR) has never been investigated. Here, we characterized the TH system in the retinas of db/db mice and highlighted regulatory processes in MIO-M1 cells. In the db/db retinas, typical functional traits and molecular signatures of DR were paralleled by a tissue-restricted reduction of TH levels. A local condition of low T3 (LT3S) was also demonstrated, which was likely to be induced by deiodinase 3 (DIO3) upregulation, and by decreased expression of DIO2 and of TH receptors. Concurrently, T3-responsive genes, including mitochondrial markers and microRNAs (miR-133-3p, 338-3p and 29c-3p), were downregulated. In MIO-M1 cells, a feedback regulatory circuit was evidenced whereby miR-133-3p triggered the post-transcriptional repression of DIO3 in a T3-dependent manner, while high glucose (HG) led to DIO3 upregulation through a nuclear factor erythroid 2-related factor 2-hypoxia-inducible factor-1 pathway. Finally, an in vitro simulated condition of early LT3S and hyperglycemia correlated with reduced markers of both mitochondrial function and stress response, which was reverted by T3 replacement. Together, the data suggest that, in the early phases of DR, a DIO3-driven LT3S may be protective against retinal stress, while, in the chronic phase, it not only fails to limit HG-induced damage, but also increases cell vulnerability likely due to persistent mitochondrial dysfunction.

Photoperiodic physiology of summer breeding birds and a search for the role of eye

Photoperiod regulation of gonadal cycles is well studied and documented in both birds and mammals. Change in photoperiod is considered as the most effective and important cue to time the initiation of the annual physiological cycles in birds. Approaching of long days (as observed in summer months), signal long-day breeding birds to initiation reproduction and other related functions. Birds and other non-mammalian vertebrates use the extraocular photoreceptors which may be present in the mediobasal hypothalamus (MBH) or associated regions to measure the photoperiodic time and so are different from mammals where only the eyes are lone photoreceptive organs. The downstream signaling involves thyroid responsive genes playing a crucial role in mediating photoperiodic signals in both birds and mammals. Role of eyes in the avian seasonal cycle has been a questionable issue with evidences both favoring and negating any role. We propose that morphological as well as physiological data argue that retinal photoreceptors can participate in gonadal cycle, at least in the quail and duck. The present review details the studies of photoneuroendocrine control of gonadal axis in birds and review evidences to decipher the role eyes in photoperiodic mediated physiologies in birds.

A Review of Thyroid Dysfunction Due to COVID-19

Coronavirus disease 2019 (COVID-19) affects thyroid function. These changes are due to the direct impact of the virus on thyroid cells via angiotensin-converting-enzyme 2 (ACE2) receptors, inflammatory reaction, apoptosis in thyroid follicular cells, suppression of hypothalamus-pituitarythyroid axis, an increase in activity of adrenocortical axis, and excess cortisol release due to cytokine storm of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Euthyroid sick syndrome (ESS), thyroiditis, clinical and subclinical hypothyroidism, central hypothyroidism, exacerbation of underlying autoimmune thyroid disease, and clinical and subclinical hyperthyroidism can be associated with coronavirus. Adjuvants in coronavirus vaccines induce autoimmune/inflammatory syndrome known as vaccine adjuvants (ASIA) syndrome. Thyroiditis and Graves' disease have been reported to be associated with ASIA syndrome after some coronavirus vaccinations. Some coronavirus medications, such as hydroxychloroquine, monoclonal antibodies, lopinavir/ritonavir, remdesivir, naproxen, anticoagulants, and glucocorticoids can also affect thyroid tests, and correct diagnosis of thyroid disorders will be more difficult. Changes in thyroid tests may be one of the most important manifestations of COVID-19. These changes can be confusing for clinicians and can lead to inappropriate diagnoses and decisions. Prospective studies should be conducted in the future to increase epidemiological and clinical data and optimize the management of thyroid dysfunctions in patients with COVID-19.

The ageing thyroid: implications for longevity and patient care

Thyroid hormones have vital roles in development, growth and energy metabolism. Within the past two decades, disturbances in thyroid hormone action have been implicated in ageing and the development of age-related diseases. This Review will consider results from biomedical studies that have identified the importance of precise temporospatial regulation of thyroid hormone action for local tissue maintenance and repair. Age-related disturbances in the maintenance of tissue homeostasis are thought to be important drivers of age-related disease. In most iodine-proficient human populations without thyroid disease, the mean, median and 97.5 centile for circulating concentrations of thyroid-stimulating hormone are progressively higher in adults over 80 years of age compared with middle-aged (50-59 years) and younger (20-29 years) adults. This trend has been shown to extend into advanced ages (over 100 years). Here, potential causes and consequences of the altered thyroid status observed in old age and its association with longevity will be discussed. In about 5-20% of adults at least 65 years of age, thyroid-stimulating hormone concentrations are elevated but circulating concentrations of thyroid hormone are within the population reference range, a condition referred to as subclinical hypothyroidism. Results from randomized clinical trials that have tested the clinical benefit of thyroid hormone replacement therapy in older adults with mild subclinical hypothyroidism will be discussed, as well as the implications of these findings for screening and treatment of subclinical hypothyroidism in older adults.

Selenium-More than Just a Fortuitous Sulfur Substitute in Redox Biology

Living organisms use selenium mainly in the form of selenocysteine in the active site of oxidoreductases. Here, selenium's unique chemistry is believed to modulate the reaction mechanism and enhance the catalytic efficiency of specific enzymes in ways not achievable with a sulfur-containing cysteine. However, despite the fact that selenium/sulfur have different physicochemical properties, several selenoproteins have fully functional cysteine-containing homologues and some organisms do not use selenocysteine at all. In this review, selected selenocysteine-containing proteins will be discussed to showcase both situations: (i) selenium as an obligatory element for the protein's physiological function, and (ii) selenium presenting no clear advantage over sulfur (functional proteins with either selenium or sulfur). Selenium's physiological roles in antioxidant defence (to maintain cellular redox status/hinder oxidative stress), hormone metabolism, DNA synthesis, and repair (maintain genetic stability) will be also highlighted, as well as selenium's role in human health. Formate dehydrogenases, hydrogenases, glutathione peroxidases, thioredoxin reductases, and iodothyronine deiodinases will be herein featured.

Transcriptomic Changes in Response to Form of Selenium on the Interferon-Tau Signaling Mechanism in the Caruncular Tissue of Beef Heifers at Maternal Recognition of Pregnancy

We have reported that selenium (Se) provided to grazing beef cattle in an inorganic (ISe) form versus a 1:1 mixture (MIX) of inorganic and organic (OSe) forms affects cholesterol biosynthesis in the corpus luteum (CL), the abundance of interferon tau (IFNτ) and progesterone (P4)-induced mRNAs in the caruncular (CAR) tissue of the endometrium, and conceptus length at maternal recognition of pregnancy (MRP). In this study, beef heifers were supplemented with a vitamin-mineral mix containing 35 ppm Se as ISe or MIX to achieve a Se-adequate status. Inseminated heifers were killed at MRP (d 17, n = 6 per treatment) for tissue collection. In CAR samples from MIX versus ISe heifers, qPCR revealed that mRNA encoding the thyroid regulating DIO2 and DIO3 was decreased (p < 0.05) and a complete transcriptomic analysis revealed effects on the interferon JAK-STAT1/2 pathway, including decreased expression of mRNAs encoding the classical interferon stimulated genes IFIT1, IFIT2, IFIT3, IRF1, IRF9, ISG15, OAS2, and RSAD2 (p < 0.05). Treatment also affected the abundance of mRNAs contributing to the immunotolerant environment (p < 0.05). In combination, these findings suggest more advanced preparation of the CAR and developing conceptus for implantation and to evade immune rejection by the maternal system in MIX- vs. ISe-treated heifers.

Demonstration of the Formation of a Selenocysteine Selenenic Acid through Hydrolysis of a Selenocysteine Selenenyl Iodide Utilizing a Protective Molecular Cradle

Selenocysteine selenenic acids (Sec-SeOHs) and selenocysteine selenenyl iodides (Sec-SeIs) have long been recognized as crucial intermediates in the catalytic cycle of glutathione peroxidase (GPx) and iodothyronine deiodinase (Dio), respectively. However, the observation of these reactive species remained elusive until our recent study, where we successfully stabilized Sec-SeOHs and Sec-SeIs using a protective molecular cradle. Here, we report the first demonstration of the chemical transformation from a Sec-SeI to a Sec-SeOH through alkaline hydrolysis. A stable Sec-SeI derived from a selenocysteine methyl ester was synthesized using the protective cradle, and its structure was determined by crystallographic analysis. The alkaline hydrolysis of the Sec-SeI at -50 °C yielded the corresponding Sec-SeOH in an 89% NMR yield, the formation of which was further confirmed by its reaction with dimedone. The facile and nearly quantitative conversion of the Sec-SeI to the Sec-SeOH not only validates the potential involvement of this process in the catalytic mechanism of Dio, but also highlights its utility as a method for producing a Sec-SeOH.

Critically ill patients: Histopathological evidence of thyroid dysfunction

PURPOSE

Critical illness is characterized by severe biphasic physical and metabolic stress as result of systemic inflammatory response syndrome and/or multiple organ dysfunction syndrome, and is frequently associated with non-thyroidal illness. Purpose of this study is to better understand the cytomorphological basis of NTI by performing histopathological examinations of thyroid gland on autopsies of patients who died from critical illness.

METHODS

Histopathological examination of thyroid gland of 58 critically ill patients was performed in our hospital. The cases included 24 cases of burn injury, 24 cases of traumatic brain injury, and 10 cases of cerebral stroke. Thyroid samples obtained during autopsy were preserved in formol saline and stained with hematoxylin and eosin. The sections were visualized under light microscopy.

RESULTS

Out of 58 cases examined, 21 patients showed normal thyroid findings, and rest of the cases had unusual thyroid findings in the histopathological study. The principal finding was distortion of thyroid follicular architecture. Other findings include mononuclear cell infiltration, clumping of thyroglobulin, and exhaustion of thyroid follicles.

CONCLUSION

Critical illness produces metabolically damaging effects on thyroid gland, which functionally corresponds to a state of low T3 syndrome. These changes are more pronounced in BI and cerebral stroke than in TBI.

Repeated toluene and cyclohexane inhalation produces differential effects on HPA and HPT axes in adolescent male rats

Misused volatile solvents typically contain toluene (TOL) as the main psychoactive ingredient. Cyclohexane (CHX) can also be present and is considered a safer alternative. Solvent misuse often occurs at early stages of life, leading to permanent neurobehavioral impairment and growth retardation. However, a comprehensive examination of the effects of TOL and CHX on stress regulation and energy balance is lacking. Here, we compared the effect of a binge-pattern exposure to TOL or CHX (4,000 or 8,000 ppm) on body weight, food intake, the hypothalamus-pituitary-adrenal (HPA) and hypothalamus-pituitary-thyroid (HPT) axes in male adolescent Wistar rats. At 8,000 ppm, TOL decreased body weight gain without affecting food intake. In addition, TOL and CHX altered the HPA and HPT axes' function in a solvent- and concentration-dependent manner. The highest TOL concentration produced HPA axis hyperactivation in animals not subjected to stress, which was evidenced by increased corticotropin-releasing-factor (CRF) release from the median eminence (ME), elevated adrenocorticotropin hormone (ACTH) and corticosterone serum levels, and decreased CRF mRNA levels in the hypothalamic paraventricular nucleus (PVN). TOL (8,000 ppm) also increased triiodothyronine (T3) serum levels, decreased pro-thyrotropin-releasing-hormone (pro-TRH) mRNA transcription in the PVN, pro-TRH content in the ME, and serum thyroid stimulating hormone (TSH) levels. CHX did not affect the HPA axis. We propose that the increased HPT axis activity induced by TOL can be related to the impaired body weight gain associated with inhalant misuse. These findings may contribute to a better understanding of the effects of the misused solvents TOL and CHX.

Biological variation in thyroid function tests in older adults and clinical implications

OBJECTIVE

Interpreting thyroid function tests can be challenging due to inherent variation, and the need for tests rises with age. While age-related changes in thyrotropin (TSH) levels are known, the biological variation in older adults remains unclear.

DESIGN

We recruited nineteen 65-99-year-old (older adults) without thyroid disease for monthly blood sampling for 1 year.

PATIENTS AND MEASUREMENTS

Serum was stored at -20C°, and TSH, total thyroxine (TT4) and total triiodothyronine (TT3) were analysed in random order in a single batch for each participant. Results were compared to test results from 15 euthyroid men aged 24-53 years (younger adults) collected previously using a similar methodology.

RESULTS

Interindividual coefficients of variation in older/younger adults were 46.7%/44.0% for TSH, 12.7%/19.5% for TT4 and 14.6%/22.4% for TT3. Intraindividual coefficients of variation (CVI ) were 19.0%/25.4% for TSH, 5.5%/10.8% for TT4 and 6.9%/13.2% for TT3. The index of individuality was below 0.6 for all hormones in all age groups. The number of samples required to determine the homoeostatic set-point at 10% precision in older adults was 14-21 for TSH and 2 for TT4 and TT3. TT4 in older adults was the only parameter in any group with comparable CVI between individuals (p = .22).

CONCLUSIONS

CVI for TT4 and TT3 was halved in older compared to younger adults with two tests of TT4 needed to describe the individual set-point. Similar CVI between older adults caused TT4 to provide a reliable estimate of thyroid function, and the added value of measuring thyroxine could improve clinical practice.