Circadian rhythm of TSH levels in subjects with Alzheimer's disease (AD)

The role of thyroid-stimulating hormone in regulating lipid metabolism: Implications for body-brain communication

Thyroid-stimulating hormone (TSH) is a pituitary hormone that stimulates the thyroid gland to produce and release thyroid hormones, primarily thyroxine and triiodothyronine. These hormones are key players in body-brain communication, influencing various physiological processes, including the regulation of metabolism (both peripheral and central effects), feedback mechanisms, and lipid metabolism. Recently, the increasing incidence of abnormal lipid metabolism has highlighted the link between thyroid function and lipid metabolism. Evidence suggests that TSH can affect all bodily systems through body-brain communication, playing a crucial role in growth, development, and the regulation of various physiological systems. Lipids serve dual purposes: they are involved in energy storage and metabolism, and they act as vital signaling molecules in numerous cellular activities, maintaining overall human health or contributing to various diseases. This article reviews the role of TSH in regulating lipid metabolism via body-brain crosstalk, focusing on its implications for common lipid metabolism disorders such as obesity, atherosclerosis, nonalcoholic fatty liver disease, neuropsychiatric disorders (including Alzheimer's disease, Parkinson's disease, multiple sclerosis, epilepsy, and depression), and cerebrovascular disorders such as stroke.

Thyroid dysfunction and Alzheimer's disease, a vicious circle

Recently, research into the link between thyroid dysfunction and Alzheimer's disease (AD) remains a current topic of interest. Previous research has primarily concentrated on examining the impact of thyroid dysfunction on the risk of developing AD, or solely explored the mechanisms of interaction between hypothyroidism and AD, a comprehensive analysis of the mechanisms linking thyroid dysfunction, including hyperthyroidism and hypothyroidism, to Alzheimer's disease (AD) still require further elucidation. Therefore, the aim of this review is to offer a thorough and comprehensive explanation of the potential mechanisms underlying the causal relationship between thyroid dysfunction and AD, highlighting the existence of a vicious circle. The effect of thyroid dysfunction on AD includes neuron death, impaired synaptic plasticity and memory, misfolded protein deposition, oxidative stress, and diffuse and global neurochemical disturbances. Conversely, AD can also contribute to thyroid dysfunction by affecting the stress repair response and disrupting pathways involved in thyroid hormone (TH) production, transport, and activation. Furthermore, this review briefly discusses the role and significance of utilizing the thyroid as a therapeutic target for cognitive recovery in AD. By exploring potential mechanisms and therapeutic avenues, this research contributes to our understanding and management of this devastating neurodegenerative disease.

Thyroid-stimulating hormone-thyroid hormone signaling contributes to circadian regulation through repressing clock2/npas2 in zebrafish

Thyroid-stimulating hormone (TSH) is important for the thyroid gland, development, growth, and metabolism. Defects in TSH production or the thyrotrope cells within the pituitary gland cause congenital hypothyroidism (CH), resulting in growth retardation and neurocognitive impairment. While human TSH is known to display rhythmicity, the molecular mechanisms underlying the circadian regulation of TSH and the effects of TSH-thyroid hormone (TH) signaling on the circadian clock remain elusive. Here we show that TSH, thyroxine (T4), triiodothyronine (T3), and tshba display rhythmicity in both larval and adult zebrafish and tshba is regulated directly by the circadian clock via both E'-box and D-box. Zebrafish tshba-/- mutants manifest congenital hypothyroidism, with the characteristics of low levels of T4 and T3 and growth retardation. Loss or overexpression of tshba alters the rhythmicity of locomotor activities and expression of core circadian clock genes and hypothalamic-pituitary-thyroid (HPT) axis-related genes. Furthermore, TSH-TH signaling regulates clock2/npas2 via the thyroid response element (TRE) in its promoter, and transcriptome analysis reveals extensive functions of Tshba in zebrafish. Together, our results demonstrate that zebrafish tshba is a direct target of the circadian clock and in turn plays critical roles in circadian regulation along with other functions.

The disturbance of thyroid-associated hormone and its receptors in brain and blood circulation existed in the early stage of mouse model of Alzheimer's disease

BACKGROUND

Studies showed that thyroid function plays an important role in the pathology of Alzheimer's disease (AD). However, changes in brain thyroid hormone and related receptors in the early stage of AD were rarely reported. The aim of this study was to explore the relationship between the early stage of AD and local thyroid hormone and its receptors in the brain.

METHODS

The animal model was established by stereotactic injection of okadaic acid (OA) into hippocampal region for the experiment, and 0.9% NS for the control. Blood sample from each mouse was collected and then the mice were sacrificed and the brain tissue was collected for detecting free triiodothyronine (FT3), free thyroid hormone (FT4), and thyroid-stimulating hormone (TSH), thyrotropin-releasing hormone (TRH) and phosphorylated tau, amyloid-β (Aβ) and thyroid hormone receptors (THRs) in the hippocampus of the mice were detected as well.

RESULTS

Enzyme-linked immunosorbent assay showed that compared with the control, FT3, FT4, TSH and TRH in brain were significantly increased in the experimental group; in the serum, FT4, TSH and TRH were increased, while FT3 had no change; western blot analysis indicated that the expression of THR α and β in the hippocampus of the experimental group was significantly higher than that of the control.

CONCLUSION

Based on the results of this study, a mouse AD model can be established successfully by injecting a small dose of OA into the hippocampus. We speculate that early AD brain and circulating thyroid dysfunction may be an early local and systemic stress repair response.

Thyroid hormone levels in Alzheimer disease: a systematic review and meta-analysis

BACKGROUND AND OBJECTIVE

Thyroid hormone (TH) disturbances are perceived to contribute to cognitive impairment and dementia. However, there is no consensus on the association between TH levels and Alzheimer Disease (AD). In this study, we aimed to compare serum and cerebrospinal fluid (CSF) TH levels in AD patients to controls by performing a meta-analysis.

METHODS

We systematically searched online databases for papers comparing CSF or serum TH levels in AD patients to controls, and performed a meta-analysis on the extracted data.

RESULTS

Out of 1604 records identified, 32 studies were included. No significant difference in serum TSH (standardized mean difference (SMD): -0.03; 95% confidence interval (CI): -0.22-0.16), total T4 (SMD: 0.10; 95% CI: -0.29-0.49), and free T4 (SMD: 0.25; 95% CI: -0.16-0.69) levels were observed. However, there was significantly lower serum total T3 (SMD: -0.56; 95%CI: -0.97 to -0.15) and free T3 (SMD: -0.47; 95%CI: -0.89 to -0.05) levels in AD group compared to controls. Subgroup analyses on studies including only euthyroid patients did not show any significant difference in either of the thyroid hormone levels. Also, no significant difference in CSF total T4 and total T3/total T4 ratios but significantly lower CSF total T3 (SMD: -2.45; 95% CI: -4.89 to -0.02) in AD group were detected.

CONCLUSION

Serum total and free T3 and CSF total T3 levels are significantly lower in AD patients compared to controls. The temporality of changes in thyroid hormone levels and AD development should be illustrated by further longitudinal studies.

The Association Between Thyroid Diseases and Alzheimer's Disease in a National Health Screening Cohort in Korea

Objectives

Thyroid dysfunction is linked to an increased risk of cognitive impairment. However, studies on the relationships between thyroid diseases and Alzheimer's disease (AD) have reported conflicting results. We investigated the associations between several thyroid diseases and AD in a nested case-control study.

Methods

A total of 1,977 participants with AD were identified by claims data from 2002-2015 among a random sample of half a million people in the Korean National Health Insurance database. We recruited 16,473 age- and sex-matched (1:4 ratio) control participants and applied conditional logistic regression to estimate the relationships between thyroid diseases and AD, with adjustments for potential confounders, such as basic demographics, lifestyle factors, and various medical conditions or comorbidities.

Results

The prevalence rates of hypothyroidism (odds ratio [OR]=1.14, 95% confidence interval [CI]=1.00-1.30), thyroiditis (OR=1.22, 95% CI=1.05-1.40), and hyperthyroidism (OR=1.13, 95% CI=1.01-1.28) were significantly higher in participants with AD than in control participants after adjustment for confounders.

Conclusion

In this large national sample, we found significant relationships between several thyroid diseases and AD. Despite of the need for further investigation, these findings could better support to appreciate the pathophysiology of AD.

Association between thyroid function and Alzheimer's disease: A systematic review

Alterations in metabolic parameters have been associated with an increased risk of dementia, among which thyroid function has gained great importance in Alzheimer's disease (AD) pathology in recent years. However, it remains unclear whether thyroid dysfunctions could influence and contribute to the beginning and/or progression of AD or if it results from AD. This systematic review was conducted to examine the association between thyroid hormone (TH) levels and AD. Medline, ISI Web of Science, EMBASE, Cochrane library, Scopus, Scielo, and LILACS were searched, from January 2010 to March 2020. A total of 17 articles were selected. The studies reported alterations in TH and circadian rhythm in AD patients. Behavior, cognition, cerebral blood flow, and glucose consumption were correlated with TH deficits in AD patients. Whether thyroid dysfunctions and AD have a cause-effect relationship was inconclusive, however, the literature was able to provide enough data to corroborate a relationship between TH and AD. Although further studies are needed in this field, the current systematic review provides information that could help future investigations.

Analysis of basal serum TSH, FT3, and FT4 levels based on age, sampling time in women with infertility

Background

To analyze the characteristics of basal thyroid hormone levels in infertile women consulting for assisted reproductive technology (ART) treatment.

Methods

This was a retrospective study. Serum TSH, FT3 and FT4 levels of women seeking ART consultation were tested routinely. Analyses were performed based on age and sampling time. One-way ANOVA or Kruskal-Wallis rank sum test was used to compare the continuous data among the groups, and the chi-square test or Fisher’s exact test was used to compare categorical data where appropriate.

Results

A total of 6426 women were initially included in the study. After exclusion criteria were applied, the remaining 4126 women were categorized into different groups. The prevalence of subclinical hypothyroidism significantly decreased with age and sampling time, from 21.09 to 11.91% and from 28.57 to 10.67%, respectively (P < 0.001, respectively). Mean serum TSH, FT3, and FT4 levels decreased significantly with age (P = 0.017, < 0.001, < 0.001, respectively). In the context of sampling time, TSH levels from early in the morning were significantly higher (P < 0.001), while FT4 and FT3 levels were similar in different groups (P = 0.258, 0.300, respectively).

Conclusions

The prevalence of subclinical hypothyroidism significantly decreased with increasing age and sampling time, as did the serum TSH levels. Even though, the establishment of reference interval of TSH level based on age or sampling time was not recommended. Full consideration of age and sampling time should be carefully taken before initiation of treatment.

Actigraphy for Assessing Light Effects on Sleep and Circadian Activity Rhythm in Alzheimer's Dementia: A Narrative Review

Background:

Alzheimer's Disease (AD) is often accompanied by severe sleep problems and circadian rhythm disturbances which may to some extent be attributed to a dysfunction in the biological clock. The 24-h light/dark cycle is the strongest Zeitgeber for the biological clock. People with AD, however, often live in environments with inappropriate photic Zeitgebers. Timed bright light exposure may help to consolidate sleep- and circadian rest/activity rhythm problems in AD, and may be a low-risk alternative to pharmacological treatment.

Objective & Method:

In the present review, experts from several research disciplines summarized the results of twenty-seven light intervention studies which used wrist actigraphy to measure sleep and circadian activity in AD patients.

Results:

Taken together, the findings remain inconclusive with regard to beneficial light effects. However, the considered studies varied substantially with respect to the utilized light intervention, study design, and usage of actigraphy. The paper provides a comprehensive critical discussion of these issues.

Conclusion:

Fusing knowledge across complementary research disciplines has the potential to critically advance our understanding of the biological input of light on health and may contribute to architectural lighting designs in hospitals, as well as our homes and work environments.

The effects of thyroid hormones on memory impairment and Alzheimer's disease

Thyroid hormones (THs) have a wide and important range of effects within the central nervous system beginning from fetal life and continuing throughout the adult life. Thyroid disorders are one of the major causes of cognitive impairment including Alzheimer's disease (AD). Several studies in recent years have indicated an association between hypothyroidism or hyperthyroidism and AD. Despite available evidence for this association, it remains unclear whether thyroid dysfunction results from or contributes to the progression of AD. This review discusses the role of THs in learning and memory and summarizes the studies that have linked thyroid function and AD. Eventually, we elaborate how THs may be effective in treating AD by putting forward potential mechanisms.

The regulation of pituitary-thyroid abnormalities by peripheral administration of levothyroxine increased brain-derived neurotrophic factor and reelin protein expression in an animal model of Alzheimer’s disease

Alzheimer’s disease (AD) is associated with decreased serum levels of thyroid hormones (THs), increased levels of thyroid-stimulating hormone (TSH), and decreased protein expression of brain-derived neurotrophic factor (BDNF) and reelin in the hippocampus. In this study, we have evaluated the effect of subcutaneous administration of levothyroxine (L-T4) on levels of THs and TSH as well as protein expression of BDNF and reelin in AD rats. To make an animal model of AD, amyloid-beta peptide (Aβ) plus ibotenic acid were infused intrahippocampally, and rats were treated with L-T4 and (or) saline for 10 days. The levels of THs and TSH were measured by ELISA kits. Protein synthesis was detected by Western blotting method. Results have been shown that serum level of THs, BDNF, and reelin protein expression in the hippocampus were significantly decreased (P < 0.001) in AD animals and elevated significantly in AD rats treated with L-T4 (P < 0.01). Data showed that TSH level significantly decreased in AD rats treated with L-T4 (P < 0.05). These findings indicated that L-T4 increased BDNF and reelin protein expression by regulation of serum THs and TSH level in Aβ-induced AD rats.

MECHANISMS IN ENDOCRINOLOGY: Impact of isolated TSH levels in and out of normal range on different tissues

Routine treatment of thyroid cancer (TC) includes long-term suppression of TSH. The necessity of this treatment in low- and intermediate-risk patients as well as the extent of TSH suppression is currently under discussion. A literature search was performed to illustrate the role of TSH in extrathyroidal cells and to identify potential reasons for different effects of exogenously suppressed and endogenously low TSH levels. Although adverse effects of subnormal and supranormal TSH blood levels on heart and brain have not been consistently found, studies show a clear negative effect of suppressed TSH levels on bone mineral density. Experimental data also support an important role of TSH in the immune system. The ability of levothyroxine (l-T4) to regulate TSH levels and triiodothyronine levels in a physiological manner is limited. Reduction of circadian changes in TSH levels, decrease of thyroid hormone-binding proteins, prevention of potential compensatory increases of TSH levels (e.g., in old age), and unresponsiveness of TSH-producing cells to TRH on l-T4 treatment might cause adverse effects of suppressed TSH levels. In view of the adverse effects of aggressive TSH suppression, achieving the suggested levels of TSH between 0.9 and 1 mU/l in the treatment of low-to-intermediate risk TC patients appears justified.

The polymorphism of ARNTL2 (BMAL2) gene rs2306074 C>T is associated with susceptibility of Alzheimer disease in Chinese population

In the present study, we investigated whether polymorphism of ARNTL2 (BMAL2) gene rs2306074 T/C was associated with susceptibility of Alzheimer disease (AD) in Chinese population. A case-control method was employed in this study. 296 unrelated AD patients and 423 control subjects were recruited in current study. The prevalence of C carriers in BMAL2 gene rs2306074 T/C in AD patients was significantly higher than that of control subjects in both the whole sample and APOE ε 4 non-carriers (in the whole sample: χ (2) = 5.938, P = 0.012; in APOE ε 4 non-carriers: χ (2) = 9.048, P < 0.0001). In addition, both in the whole sample and APOE ε 4 non-carriers, prevalence of CC genotypes in BMAL2 gene rs2306074 of AD patients was also significantly higher than that in controls (in the whole sample: χ (2) = 5.126, P = 0.018; in APOE ε 4 non-carriers: χ (2) = 7.389, P = 0.023). However, there was no significant difference of prevalence of C carriers and CC genotypes in BMAL2 gene rs2306074 T/C between AD patients and control subjects among APOE ε 4 carriers (C carriers: χ (2) = 0.020, P = 0.900; CC genotypes: χ (2) = 0.017, P = 0.946). C carriers in BMAL2 gene rs2306074 T/C are associated with a high susceptibility of AD among APOE ε 4 non-carriers but not among APOE ε 4 carriers in Chinese population.

Effects of dietary sea cucumber saponin on the gene expression rhythm involved in circadian clock and lipid metabolism in mice during nighttime-feeding

In mammals, clock rhythms exist not only in the suprachiasmatic nucleus, which is entrained by light/dark (LD) cycles, but also in most peripheral tissues. Recent studies have revealed that most physiology and behavior are subject to well-controlled daily oscillations; similarly, metabolic state influences the diurnal rhythm too. Previous studies have indicated that dietary sea cucumber saponin (SCS) could improve glucose and lipid metabolism of rodent. However, whether SCS could affect the expression of clock genes, which is involved in lipid metabolism, is unknown at present. The aim of this study is to investigate the effects of SCS on the clock and clock-controlled genes involved in lipid metabolism. ICR male mice were divided into a control and SCS group mice (add 0.03% sea cucumber saponin to regular chow) and were fed at night (2030-0830 hours). After 2 weeks, clock genes expression in brain and liver, blood glucose, hormones, and lipid metabolic markers were analyzed. The results showed that dietary SCS caused alteration in rhythms and/or amplitudes of clock genes was more significant in brain than in liver. In addition, peroxisome proliferator-activated receptor (PPARα), sterol regulatory element binding protein-1c (SREBP-1c), together with their target genes carnitine palmitoyl transferase (CPT), and fatty acid synthase (FAS) showed marked changes in rhythm and/or amplitude in SCS group mice. These results suggested that SCS could affect the daily expression patterns of clock genes in brain and liver tissues, and alter the clock-controlled genes involved in lipid metabolism.