Thyroid hormone regulates endogenous amyloid-beta precursor protein gene expression and processing in both in vitro and in vivo models

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.

Alteration of Thyroid Hormones in Mouse Models of Alzheimer's Disease and Aging

INTRODUCTION

Aging is characterized by the deterioration of a wide range of functions in tissues and organs, and Alzheimer's disease (AD) is a neurodegenerative disease characterized by cognitive impairment. Hypothyroidism occurs when there is insufficient production of thyroid hormones (THs) by the thyroid. The relationship between hypothyroidism and aging as well as AD is controversial at present.

METHODS

We established an animal model of AD (FAD4T) with mutations in the APP and PSEN1 genes, and we performed a thyroid function test and RNA sequencing (RNA-Seq) of the thyroid from FAD4T and naturally aging mice. We also studied gene perturbation correlation in the FAD4T mouse thyroid, bone marrow, and brain by further single-cell RNA sequencing (scRNA-seq) data of the bone marrow and brain.

RESULTS

In this study, we found alterations in THs in both AD and aging mice. RNA-seq data showed significant upregulation of T-cell infiltration- and cell proliferation-related genes in FAD4T mouse thyroid. In addition, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses revealed that upregulated genes were enriched in the functional gene modules of activation of immune cells. Downregulated energy metabolism-related genes were prominent in aging thyroids, which reflected the reduction in THs. GSEA showed a similar enrichment tendency in both mouse thyroids, suggesting their analogous inflammation state. In addition, the regulation of leukocyte activation and migration was a common signature between the thyroid, brain, and bone marrow of FAD4T mice.

CONCLUSIONS

Our findings identified immune cell infiltration of the thyroid as the potential underlying mechanism of the alteration of THs in AD and aging.

Sex and Sleep Disruption as Contributing Factors in Alzheimer's Disease

Alzheimer's disease (AD) affects more women than men, with women throughout the menopausal transition potentially being the most under researched and at-risk group. Sleep disruptions, which are an established risk factor for AD, increase in prevalence with normal aging and are exacerbated in women during menopause. Sex differences showing more disrupted sleep patterns and increased AD pathology in women and female animal models have been established in literature, with much emphasis placed on loss of circulating gonadal hormones with age. Interestingly, increases in gonadotropins such as follicle stimulating hormone are emerging to be a major contributor to AD pathogenesis and may also play a role in sleep disruption, perhaps in combination with other lesser studied hormones. Several sleep influencing regions of the brain appear to be affected early in AD progression and some may exhibit sexual dimorphisms that may contribute to increased sleep disruptions in women with age. Additionally, some of the most common sleep disorders, as well as multiple health conditions that impair sleep quality, are more prevalent and more severe in women. These conditions are often comorbid with AD and have bi-directional relationships that contribute synergistically to cognitive decline and neuropathology. The association during aging of increased sleep disruption and sleep disorders, dramatic hormonal changes during and after menopause, and increased AD pathology may be interacting and contributing factors that lead to the increased number of women living with AD.

Association of Thyroid Hormone Medication Adherence With Risk of Dementia

CONTEXT

Recent studies suggest that hypothyroidism is a risk factor for dementia. Based on existing literature, it is unclear if treatment or compliance to treatment is associated with decreased dementia risk in hypothyroidism patients.

OBJECTIVE

To determine the association between thyroid hormone medication adherence and risk of dementia.

METHODS

A population-based cohort study following patients with newly diagnosed hypothyroidism until incidence of dementia, death, or December 31, 2020, whichever came earliest. The study comprised 41 554 older adults age 50 and above with newly diagnosed hypothyroidism between 2004 and 2008 who underwent health screening. Risk of dementia was evaluated using multivariable-adjusted Cox proportional hazards regression.

RESULTS

Among 5188 men (12.5%) and 36 366 (87.5%) women, 2120 cases of incident dementia were identified. After stratification of the participants according to the interquartile range of the medication possession ratio (MPR), the fourth quartile (highest adherence) showed a 14% lower risk of overall dementia compared with the first quartile (lowest adherence) (adjusted hazard ratio 0.86; 95% CI 0.76-0.97). No consistent association was observed between thyroid hormone medication adherence and vascular dementia. After a dichotomous stratification of the MPR, higher MPR (≥0.8) showed a lower risk of Alzheimer disease (adjusted hazard ratio 0.91; 95% CI 0.84-0.99) than lower MPR (<0.8).

CONCLUSION

Better adherence to thyroid hormone medication may be beneficial in preventing incident dementia, especially Alzheimer disease, in older adults with newly diagnosed hypothyroidism. Along with the resolution of hypothyroid symptoms and the reduction of cardiovascular risk factors, the reduction of dementia risk may be another potential benefit of thyroid hormone medication.

Impact of thyroid hormone perturbations in adult mice: brain weight and blood vessel changes, gene expression variation, and neurobehavioral outcomes

Mouse models of hyper- and hypothyroidism were used to examine the effects of thyroid hormone (TH) dyshomeostasis on the aging mammalian brain. 13-14 month-old mice were treated for 4months with either levothyroxine (hyperthyroid) or a propylthiouracil and methimazole combination (PTU/Met; hypothyroid). Hyperthyroid mice performed better on Morris Water Maze than control mice, while hypothyroid mice performed worse. Brain weight was increased in thyroxine-treated, and decreased in PTU/Met-treated animals. The brain weight change was strongly correlated with circulating and tissue T4. Quantitative measurements of microvessels were compared using digital neuropathologic methods. There was an increase in microvessel area in hyperthyroid mice. Hypothyroid mice showed a trend for elevated glial fibrillary acidic protein-immunoreactive astrocytes, indicating an increase in neuroinflammation. Gene expression alterations were associated with TH perturbation and astrocyte-expressed transcripts were particularly affected. For example, expression of Gli2 and Gli3, mediators in the Sonic Hedgehog signaling pathway, were strongly impacted by both treatments. We conclude that TH perturbations produce robust neurobehavioral, pathological, and brain gene expression changes in aging mouse models.

Alzheimer's Disease and Impaired Bone Microarchitecture, Regeneration and Potential Genetic Links

Alzheimer's Disease (AD) and osteoporosis are both age-related degenerative diseases. Many studies indicate that these two diseases share common pathogenesis mechanisms. In this review, the osteoporotic phenotype of AD mouse models was discussed, and shared mechanisms such as hormonal imbalance, genetic factors, similar signaling pathways and impaired neurotransmitters were identified. Moreover, the review provides recent data associated with these two diseases. Furthermore, potential therapeutic approaches targeting both diseases were discussed. Thus, we proposed that preventing bone loss should be one of the most important treatment goals in patients with AD; treatment targeting brain disorders is also beneficial for osteoporosis.

Neuropsychiatric Manifestations of Thyroid Diseases

Thyroid disorders are known to cause neuropsychiatric manifestations. Various neuropsychiatric manifestations are depression, dementia, mania, and autoimmune Hashimoto encephalopathy. Numerous investigations carried out in the previous 50-60 years have been evaluated critically. The pathophysiology of neuropsychiatric symptoms of thyroid diseases is described in the current study and its link with autoimmune Hashimoto encephalopathy is also discussed. Furthermore, this paper also describes the association between thyroid-stimulating hormones and cognitive impairment. Hypothyroidism is associated with depression and mania, and hyperthyroidism is linked with dementia and mania. The association between Graves' disease and various mental disorders such as depressive and anxiety disorders is also discussed. The aim of this study is to review the relationship between various neuropsychiatric disorders and thyroid diseases. A literature search from the PubMed database to find various neuropsychiatric manifestations of thyroid disorders in the adult population was conducted. According to the review of the studies, cognitive impairment can result from thyroid disease. It has not been possible to demonstrate how hyperthyroidism can hasten the process of developing dementia. However, subclinical hyperthyroidism, thyroid-stimulating hormone (TSH) levels below the normal range, and high free thyroxine (T4) levels all raise the risk of dementia in the elderly. Additionally, the potential mechanisms underlying this association have been examined. A quick summary of the research on mania as a clinical symptom of hypothyroidism and its likely causes and pathogenesis is also reviewed. There is no dearth of evidence that describes various neuropsychiatric manifestation in thyroid disorders.

Triiodothyronine Treatment reverses Depression-Like Behavior in a triple-transgenic animal model of Alzheimer's Disease

Alzheimer disease's (AD) is a neurodegenerative disorder characterized by cognitive and behavioral impairment. The central nervous system is an important target of thyroid hormones (TH). An inverse association between serum triiodothyronine (T3) levels and the risk of AD symptoms and progression has been reported. We investigated the effects of T3 treatment on the depression-like behavior in male transgenic 3xTg-AD mice. Animals were divided into 2 groups treated with daily intraperitoneal injections of 20 ng/g of body weight (b.w.) L-T3 (T3 group) or saline (vehicle, control group). The experimental protocol lasted 21 days, and behavioral tests were conducted on days 18-20. At the end of the experiment, the TH profile and hippocampal gene expression were evaluated. The T3-treated group significantly increased serum T3 and decreased thyroxine (T4) levels. When compared to control hippocampal samples, the T3 group exhibited attenuated glycogen synthase kinase-3 (GSK3), metalloproteinase 10 (ADAM10), amyloid-beta precursor-protein (APP), serotonin transporter (SERT), 5HT1A receptor, monocarboxylate transporter 8 (MCT8) and bone morphogenetic protein 7 (BMP-7) gene expression, whereas augmented superoxide dismutase 2 (SOD2) and Hairless gene expression. T3-treated animals also displayed reduced immobility time in both the tail suspension and forced swim tests, and in the latter presented a higher latency time compared to the control group. Therefore, our findings suggest that in an AD mouse model, T3 supplementation promotes improvements in depression-like behavior, through the modulation of the serotonergic related genes involved in the transmission mediated by 5HT1A receptors and serotonin reuptake, and attenuated disease progression.

Association of serum thyroid hormone levels with positron emission tomography imaging in non-demented older adults

BACKGROUND

Although increasing evidence indicates that even variations in normal range thyroid function are associated with Alzheimer's disease (AD), the association between serum thyroid hormone levels within the reference range and AD biomarkers remains unclear. This study examined whether variations in thyroid hormones within the reference range are associated with brain amyloid burden and cortical glucose metabolism in older adults without dementia.

METHODS

One hundred and two non-demented older adults underwent ¹¹ C-Pittsburgh Compound B positron emission tomography (PiB-PET), ¹⁸ F-fluorodeoxyglucose (FDG)-PET, and measurement of serum thyroid-stimulating hormone (TSH), free triiodothyronine (T3), and free thyroxine (T4) levels. The discrimination between PiB-negative and PiB-positive subgroup was made on the basis of a subject's cortical uptake value ratio greater than 1.4. The association of serum thyroid hormone levels with global PiB or FDG uptake, and PiB or FDG uptake in each region of interest, including frontal and temporoparietal lobes and posterior cingulate gyrus, was analysed using a multiple regression model with adjustment for covariates, including age, gender, years of education, apolipoprotein E4 status or PiB uptake value.

RESULTS

In the PiB-positive subgroup, the serum TSH levels positively associated with the global FDG uptake (β = 0.471, P = 0.003) and FDG uptake in the frontal and temporoparietal lobes (β = 0.466, P = 0.003, β = 0.394, P = 0.012, respectively); the serum-free T3 levels negatively associated with the FDG uptake in the temporoparietal lobe and posterior cingulate region (β = -0.351, P = 0.033, β = -0.544, P = 0.002, respectively). The PiB-negative subgroup showed no significant associations. The serum thyroid hormone levels did not correlate with the global PiB uptake and PiB uptake in each region.

CONCLUSIONS

The variations in the thyroid hormones within the reference ranges are associated with glucose metabolism, particularly in the specific regions affected by the neuropathologic changes of AD, in non-demented older adults with brain amyloid burden.

Optimizations for Computing Relatedness in Biomedical Heterogeneous Information Networks: SemNet 2.0

Literature-based discovery (LBD) summarizes information and generates insight from large text corpuses. The SemNet framework utilizes a large heterogeneous information network or “knowledge graph” of nodes and edges to compute relatedness and rank concepts pertinent to a user-specified target. SemNet provides a way to perform multi-factorial and multi-scalar analysis of complex disease etiology and therapeutic identification using the 33+ million articles in PubMed. The present work improves the efficacy and efficiency of LBD for end users by augmenting SemNet to create SemNet 2.0. A custom Python data structure replaced reliance on Neo4j to improve knowledge graph query times by several orders of magnitude. Additionally, two randomized algorithms were built to optimize the HeteSim metric calculation for computing metapath similarity. The unsupervised learning algorithm for rank aggregation (ULARA), which ranks concepts with respect to the user-specified target, was reconstructed using derived mathematical proofs of correctness and probabilistic performance guarantees for optimization. The upgraded ULARA is generalizable to other rank aggregation problems outside of SemNet. In summary, SemNet 2.0 is a comprehensive open-source software for significantly faster, more effective, and user-friendly means of automated biomedical LBD. An example case is performed to rank relationships between Alzheimer’s disease and metabolic co-morbidities.

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.

Comparison of Serum Triiodothyronine with Biomarkers for Alzheimer's Disease Continuum in Euthyroid Subjects

BACKGROUND

Accumulating studies have implicated thyroid dysfunction in the pathogenesis of Alzheimer's disease (AD).

OBJECTIVE

This study aimed to explore the association between thyroid hormone (TH) levels and cerebrospinal fluid (CSF) biomarkers for AD continuum among euthyroid subjects.

METHODS

In all, 93 clinically euthyroid subjects with a cognitive decline were included in this prospective cross-sectional study and were divided into groups with abnormal AD biomarkers (belonging to the "Alzheimer's continuum"; A+ patients) and those with "normal AD biomarkers" or "non-AD pathological changes" (A-patients), according to the ATN research framework classification for AD. A partial correlation analysis of serum thyroid-stimulating hormone (TSH) or TH levels with CSF biomarkers was conducted. The predictor for A+ patients was analyzed via binary logistic regressions. Finally, the diagnostic significance of individual biochemical predictors for A+ patients was estimated via receiver operating characteristic curve analysis.

RESULTS

Serum total triiodothyronine (TT3) and free triiodothyronine (FT3) levels were found to affect the levels of CSF amyloid-β (Aβ)42 and the ratios of Aβ 42/40. Further, FT3 was found to be a significant predictor for A+ via binary logistic regression modeling. Moreover, FT3 showed a high diagnostic value for A+ in euthyroid subjects.

CONCLUSION

Even in a clinical euthyroid state, low serum FT3 and TT3 levels appear to be differentially associated with AD-specific CSF changes. These data indicate that serum FT3 is a strong candidate for differential diagnosis between AD continuum and non-AD dementia, which benefits the early diagnosis and effective management of preclinical and clinical AD patients.

The Role of Age on Beta-Amyloid1-42 Plasma Levels in Healthy Subjects

Beta-amyloid (Aβ) plaques have been observed in the brain of healthy elderlies with frequencies strongly influenced by age. The aim of the study is to evaluate the role of age and other biochemical and hematological parameters on Aβ_1–42 plasma levels in cognitively and neurologically normal individuals. Two-hundred and seventy-five normal subjects stratified by age groups (<35 years, 35–65 years, and >65 years) were included in the study. Aβ_1–42 plasma levels significantly correlated with age (r_s = 0.27; p < 0.0001) in the whole sample, inversely correlated with age in the first age group (r_s = −0.25, p = 0.01), positively correlated in the second group (r_s = 0.22, p = 0.03), while there was no significant correlation in the older group (r_s = 0.02, p = 0.86). Both age (β-estimate = 0.08; p < 0.001) and cholesterol (β-estimate = 0.03; p = 0.009) were significantly associated with Aβ_1–42 plasma level in multivariable analysis. However, only the association with age survived post hoc adjustment for multiple comparisons. The different effects of age on the Aβ level across age groups should be explored in further studies to better understand the age-dependent variability. This could better define the value of plasma Aβ as a biomarker of the Alzheimer neuropathology.

Association between Hypothyroidism Onset and Alzheimer Disease Onset in Adults with Down Syndrome

Adults with Down syndrome (DS) have an exceptionally high frequency of Alzheimer disease (AD) with a wide variability in onset, from 40 to 70 years of age. Equally prevalent in DS is hypothyroidism. In this study, we sought to quantify the relationship between the two. A total of 232 adults with DS and AD were stratified into three AD onset age groups: early (<47 years), typical (48–59), and late (>59). Among patients with available data, differences in the distributions of demographics, hypothyroidism variables (presence, age of onset), thyroid function tests, thyroid autoantibodies, and APOE genotypes were assessed (e.g., chi-squared, Mann–Whitney tests). Spearman and partial Spearman correlations and ordinal logistic regression models were constructed to quantify the association between ages of AD and hypothyroidism onset with and without covariate adjustments. We observed a positive association between the ages of AD and hypothyroidism onset after accounting for APOE-Ɛ4 (correlation: 0.44, 0.24, 0.60; odds ratio: 1.09, 1.05–1.14). However, an early age of hypothyroidism onset and the presence of the APOE-Ɛ4 allele were independently associated with the early age of AD onset. Similar findings were observed when accounting for other factors. Our study provides evidence for the importance of hypothyroidism and associated pathological mechanisms for risk of AD in DS.

The Role of Thyroid Function in Alzheimer's Disease

The thyroid gland is crucial for the regulation of metabolism, growth, and development of various tissues, organs, systems, including the central nervous system. Recent studies have implicated the role of thyroid dysfunction in the etiology of Alzheimer's disease (AD), while AD leads to a significant increase in the prevalence of thyroid dysfunction. In this review, we have analyzed the role of thyroid function in the pathophysiology of AD as well as its biomarkers. The present review aims to provide encouraging targets for early screening of AD risk factors and intervention strategies.

The Role of Thyroid Hormone in Neuronal Protection

Thyroid hormone is essential for brain development and brain function in the adult. During development, thyroid hormone acts in a spatial and temporal-specific manner to regulate the expression of genes essential for normal neural cell differentiation, migration, and myelination. In the adult brain, thyroid hormone is important for maintaining normal brain function. Thyroid hormone excess, hyperthyroidism, and thyroid hormone deficiency, hypothyroidism, are associated with disordered brain function, including depression, memory loss, impaired cognitive function, irritability, and anxiety. Adequate thyroid hormone levels are required for normal brain function. Thyroid hormone acts through a cascade of signaling components: activation and inactivation by deiodinase enzymes, thyroid hormone membrane transporters, and nuclear thyroid hormone receptors. Additionally, the hypothalamic-pituitary-thyroid axis, with negative feedback of thyroid hormone on thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH) secretion, regulates serum thyroid hormone levels in a narrow range. Animal and human studies have shown both systemic and local reduction in thyroid hormone availability in neurologic disease and after brain trauma. Treatment with thyroid hormone and selective thyroid hormone analogs has resulted in a reduction in injury and improved recovery. This article will describe the thyroid hormone signal transduction pathway in the brain and the role of thyroid hormone in the aging brain, neurologic diseases, and the protective role when administered after traumatic brain injury. © 2021 American Physiological Society. Compr Physiol 11:1-21, 2021.

T1AM-TAAR1 signalling protects against OGD-induced synaptic dysfunction in the entorhinal cortex

Abnormalities in thyroid hormones (TH) availability and/or metabolism have been hypothesized to contribute to Alzheimer's disease (AD) and to be a risk factor for stroke. Recently, 3-iodothyronamine (T₁AM), an endogenous amine putatively derived from TH metabolism, gained interest for its ability to promote learning and memory in the mouse. Moreover, T₁AM has been demonstrated to rescue the β-Amyloid dependent LTP impairment in the entorhinal cortex (EC), a brain area crucially involved in learning and memory and early affected during AD. In the present work, we have investigated the effect of T₁AM on ischemia-induced EC synaptic dysfunction. In EC brain slices exposed to oxygen-glucose deprivation (OGD), we demonstrated that the acute perfusion of T₁AM (5 μM) was capable of preventing ischemia-induced synaptic depression and that this protective effect was mediated by the trace amine-associated receptor 1 (TAAR1). Moreover, we demonstrated that activation of the BDNF-TrkB signalling is required for T₁AM action during ischemia. The protective effect of T₁AM was more evident when using EC slices from transgenic mutant human APP (mhAPP mice) that are more vulnerable to the effect of OGD. Our results confirm that the TH derivative T₁AM can rescue synaptic function after transient ischemia, an effect that was also observed in a Aβ-enriched environment.

Is Hyperthyroidism a Possible Etiology of Early Onset Dementia?

Dementia, a disabling syndrome of the elderly characterized by the decline in memory and cognition, is increasing in incidence and affects not only the individual but also their family and close ones. Hyperthyroidism can mimic many other diseases and untreated hyperthyroidism can lead to adverse problems of various systems including the heart, bones, muscles, menstrual cycle, and fertility. In this article, we have tried to evaluate the association between hyperthyroidism and dementia, as well as the impact of hyperthyroidism management in the treatment and prevention of dementia. Studies available in the PubMed database have been used, excluding animal studies and including studies of adults above the age of 50. The analysis of studies reveals that thyroid dysfunction can lead to cognitive impairment. It has not been able to prove that hyperthyroidism can lead to an earlier onset of dementia. But subclinical hyperthyroidism, thyroid-stimulating hormone (TSH) levels below the normal range, and high free thyroxine (T4) levels increase the risk of dementia among the elderly. The possible mechanisms involved in this association have also been discussed. Thus, we concluded that it is essential to detect and manage hyperthyroidism at an earlier stage since hyperthyroidism increases the risk of dementia. The possibility of using antithyroid treatment in euthyroid dementia is yet to be studied extensively.

On the Etiopathogenesis and Pathophysiology of Alzheimer's Disease: A Comprehensive Theoretical Review

Alzheimers' disease (AD) is the most common cause of dementia, with an estimated 5 million new cases occurring annually. Among the elderly, AD shortens life expectancy, results in disability, decreases quality of life, and ultimately, leads to institutionalization. Despite extensive research in the last few decades, its heterogeneous pathophysiology and etiopathogenesis have made it difficult to develop an effective treatment and prevention strategy. Aging is the biggest risk factor for AD and evidence suggest that the total number of older people in the population is going to increase astronomically in the next decades. Also, there is evidence that air pollution and increasing income inequality may result in higher incidence and prevalence of AD. This makes the need for a comprehensive understanding of the etiopathogenesis and pathophysiology of the disease extremely critical. In this paper, a quintuple framework of thyroid dysfunction, vitamin D deficiency, sex hormones, and mitochondria dysfunction and oxidative stress are used to provide a comprehensive description of AD etiopathogenesis and pathophysiology. The individual role of each factor, their synergistic and genetic interactions, as well as the limitations of the framework are discussed.

On the central role of mitochondria dysfunction and oxidative stress in Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is the commonest cause of dementia, with approximately 5 million new cases occurring annually. Despite decades of research, its complex pathophysiology and etiopathogenesis presents a major hindrance to the development of an effective treatment and prevention strategy. Aging is the biggest risk factor for the development of AD, and the total number of older people in the population is going to significantly increase in the next decades, suggesting that AD incidence and prevalence is likely to increase in the future. This makes the need for a better understanding of the disease to be extremely urgent.

METHODS

A search was done by accessing PubMed/Medline, EBSCO, and PsycINFO databases. The search string used was "(dementia* OR Alzheimer's) AND (pathophysiology* OR pathogenesis)". New key terms were identified (new term included "vitamin D, thyroid hormone, mitochondria dysfunction, oxidative stress, testosterone, estrogen, melatonin, progesterone, luteinizing hormone, amyloid-β (Aβ), and hyperphosphorylated tau"). The electronic databases were searched for titles or abstracts containing these terms in all published articles between January 1, 1965, and January 31, 2019. The search was limited to studies published in English and other languages involving both animal and human subjects.

RESULTS

Mitochondria dysfunction and oxidative stress play a critical role in AD etiopathogenesis and pathophysiology.

CONCLUSION

AD treatment and prevention strategies must be geared towards improving mitochondrial function and attenuating oxidative stress.

Low serum concentration of free triiodothyronine (FT3) is associated with increased risk of Alzheimer's disease

BACKGROUND

In epidemiological studies, thyroid hormones (THs) have been associated with the risk of dementia. However, little is known of the relation between THs and risk of Alzheimer's disease (AD) or vascular dementia (VaD) in a memory clinic population.

METHODS

In a mono-center study, serum concentrations of thyroid-stimulating hormone (TSH), free thyroxine (FT4), and free triiodothyronine (FT3) were assessed in 302 patients. All patients had subjective or objective mild cognitive impairment and none received treatment with THs. Cox proportional hazards regression analyses was used to determine whether THs at baseline were associated with the risk of conversion to all-cause dementia, AD or VaD.

RESULTS

During the follow-up (mean 2.8 years), 82 (28%) of the patients progressed to dementia [AD, n = 55 (18%) and VaD, n = 17 (6%)]. Serum concentrations of TSH, FT4, and FT3 did not associate with all-cause dementia or VaD. Higher serum FT3 was associated with lower risk of conversion to AD [hazard ratio (HR) = 054; 95% confidence interval (CI): 0.32-0.92 per 1 pmol/L increase]. Furthermore, patients in the lowest serum FT3 quartile had a twofold increased risk of AD compared to those in the highest quartile (HR = 2.63; 95% CI: 1.06-6.47). These associations remained after adjustment for multiple covariates.

CONCLUSIONS

In a memory clinic population, there was an inverse, linear association between serum FT3 and risk of AD whereas THs did not associate with all-cause dementia or VaD. Further studies are needed to determine the underlying mechanisms as well as the clinical significance of these findings.

Thyroid Hormone Enhances Neurite Outgrowth in Neuroscreen 1 Cells

Objectives

Alzheimer's disease (AD) is a neurodegenerative disorder that affects millions of individuals. Moreover, hypothyroidism has been identified as one of the risk factors that may contribute to the development of AD. Here, we investigated whether there was a correlation among expression levels of proteins involved in the formation of AD lesions, neurite outgrowth, and thyroid hormone levels.

Methods

Cells were grown in media supplemented with different levels of 3,5,3'-triiodothyronine (T3) and then processed for neurite outgrowth and to prepare RNA samples. RNA samples were analysed using quantitative real-time PCR. Protein levels were measured using in cell-Western blotting analysis.

Results

By using neurite outgrowth studies, it was demonstrated that T3 treatment enhanced neurite outgrowth in NS-1 cells in a time- and dose-dependent manner. Quantitative real-time PCR studies further confirmed that NS-1 cells expressed substantial levels of TRα and significantly less TRβ, either of which could be responsible for the T3-dependent effects on neurite outgrowth. Although the overall tau protein expression was not affected in response to T3 treatment, the splicing of tau exon 10 was impacted in the direction of producing more tau molecules that excluded the exon (tau 3R).

Conclusion

The results of this study are critical not only to understand the probable link between hypothyroidism and AD but also in providing the basis for future prevention and treatment of AD in hypothyroid patients.

Hypothyroidism increases cyclooxygenase-2 levels and pro-inflammatory response and decreases cell proliferation and neuroblast differentiation in the hippocampus

The present study investigated the effects of hypothyroidism on cyclooxygenase-2 (COX-2) and pro‑inflammatory cytokines in the dentate gyrus to elucidate the roles of COX‑2 in the hypothyroid hippocampus. Hypothyroidism was induced in rats by treating with 0.03% 2‑mercapto‑1‑methyl‑imidazole dissolved in drinking water for 5 weeks. The animals were sacrificed at 12 weeks of age. Hypothyroidism rats exhibited decreased triiodothyronine and thyroxine levels in the serum, while the levels of thyroid‑stimulating hormone and the weight of thyroid glands were significantly higher in the hypothyroid rats compared with those in the vehicle‑treated group. COX‑2 immunoreactivity was significantly increased in the hippocampal CA2/3 region and the dentate gyrus compared with the vehicle‑treated group. Levels of pro‑inflammatory cytokines including interleukin (IL)‑1β, IL‑6 and tumor necrosis factor‑α were significantly higher in the hippocampal homogenates of hypothyroid rats. Cell proliferation and neuroblast differentiation based on Ki67 and doublecortin immunohistochemistry were decreased in the dentate gyrus of hypothyroid rats compared with those in the vehicle‑treated group. These results suggested that hypothyroidism‑mediated COX‑2 expression affected hippocampal plasticity by upregulating the levels of pro‑inflammatory cytokines in the hippocampus. Therefore, COX‑2 may be suggested as a candidate molecule for preventing hypothyroidism‑induced neurological side effects.

Thyroid hormone and the brain: Mechanisms of action in development and role in protection and promotion of recovery after brain injury

Thyroid hormone (TH) is essential for normal brain development and may also promote recovery and neuronal regeneration after brain injury. TH acts predominantly through the nuclear receptors, TH receptor alpha (THRA) and beta (THRB). Additional factors that impact TH action in the brain include metabolism, activation of thyroxine (T4) to triiodothyronine (T3) by the enzyme 5'-deiodinase Type 2 (Dio2), inactivation by the enzyme 5-deiodinase Type 3 (Dio3) to reverse T3 (rT3), which occurs in glial cells, and uptake by the Mct8 transporter in neurons. Traumatic brain injury (TBI) is associated with inflammation, metabolic alterations and neural death. In clinical studies, central hypothyroidism, due to hypothalamic and pituitary dysfunction, has been found in some individuals after brain injury. TH has been shown, in animal models, to be protective for the damage incurred from brain injury and may have a role to limit injury and promote recovery. Although clinical trials have not yet been reported, findings from in vitro and in vivo models inform potential treatment strategies utilizing TH for protection and promotion of recovery after brain injury.

Cognitive Function, APOE Gene Polymorphisms, and Thyroid Status Associations in Postmenopausal Women in Poland

BACKGROUND

The objective of the study was to analyze a potential association between cognitive functions and thyroid status in postmenopausal women with different polymorphisms of the apolipoprotein E gene (APOE).

METHODS

The examined population included 402 postmenopausal women from south-eastern Poland. The evaluation of cognitive functions was made with the use of the diagnostic Central Nervous System-Vital Signs equipment (Polish version). Multiplex polymerase chain reactions were performed to assess APOE polymorphisms. The thyroid hormone tests were assessed by an accredited laboratory.

RESULTS AND CONCLUSION

Lower results of cognitive functions were associated with the presence of the ε4 APOE allele in postmenopausal women. The ε4 APOE polymorphism was associated with a higher concentration of thyroid-stimulating hormone and lower concentrations of free triiodothyronine and total triiodothyronine.

Low-frequency stimulation induces a durable long-term depression in young adult hyperthyroid rats: the role of p38 mitogen-activated protein kinase and protein phosphatase 1

Long-term potentiation and long-term depression (LTD) are cellular mechanisms of learning and memory in the mammalian brain. We have previously shown that adult hyperthyroid rats showed a delay in the acquisition of a place learning task and attenuated long-term potentiation. However, changes in LTD in hyperthyroidism remain unclear. Rats were administered 0.2 mg/kg/day of L-thyroxine for 21 days starting at postnatal day 40 to induce hyperthyroidism. LTD was induced in the dentate gyrus using low-frequency stimulation (LFS) of the perforant pathway. The mRNA expressions of p38 mitogen-activated protein kinase (p38-MAPK) and protein phosphatase 1 (PP1) were evaluated using a quantitative reverse transcriptase PCR. In control rats, a standard LFS protocol induced a slight depression of the population spike (PS) amplitude during the induction phase of LTD (76±13% vs. baseline), but a small potentiation of the PS amplitude was observed in the early (107±18%) and late (111±20%) phases of LTD. Interestingly, in the hyperthyroid rats, the same LFS protocol induced a reliable LTD in the dentate gyrus of the hippocampus as evidenced by a marked depression in the PS amplitude during the induction (54±6% vs. baseline) and the early phases (56±8%) of LTD. Elevated mRNA levels of p38-MAPK and PP1 were observed in the hippocampus of the LFS-treated hyperthyroid rats compared with the hippocampus of the vehicle-treated hyperthyroid rats. No significant change in p38-MAPK or PP1 mRNA expression was observed in the euthyroid rats. The present study shows that a standard LFS protocol can induce a durable depression of synaptic strength and an upregulation of PP1 and p38-MAPK mRNA in hyperthyroid rats. We conclude that hyperthyroidism can induce molecular changes associated with degeneration of the hippocampus. The relationship between the levels of thyroid hormone and dementia requires further investigation.

Potential Link Between Proprotein Convertase Subtilisin/Kexin Type 9 and Alzheimer's Disease

Alzheimer's disease [AD] is not only the most common neurodegenerative disease but is also currently incurable. Proprotein convertase subtilisin/kexin-9 [PCSK9] is an indirect regulator of plasma low density lipoprotein [LDL] levels controlling LDL receptor expression at the plasma membrane. PCSK9 also appears to regulate the development of glucose intolerance, insulin resistance, abdominal obesity, inflammation, and hypertension, conditions that have been identified as risk factors for AD. PCSK9 levels also depend on age, sex, and ethnic background, factors associated with AD. Herein, we will review indirect evidence that suggests a link between PCSK9 levels and AD.

Associations of thyroid hormone serum levels with in-vivo Alzheimer's disease pathologies

Background

The present study investigated the relationships between thyroid hormone serum levels or thyroid-stimulating hormone (TSH) and two Alzheimer’s disease (AD)-specific biomarkers, cerebral amyloid beta (Aβ) burden and glucose metabolism, in AD-signature brain regions in cognitively normal (CN) middle-aged and older individuals.

Methods

This study assessed 148 CN individuals who received comprehensive clinical and neuropsychological assessments that included ¹¹C-Pittsburgh Compound B (PiB)-positron emission tomography (PET) scans, ¹⁸F-deoxyglucose (FDG)-PET scans, and the quantification of serum triiodothyronine (T3), free T3, free thyroxine (fT4), and TSH levels.

Results

All participants were clinically euthyroid. Independent negative associations were found between serum fT4 levels and global cerebral Aβ deposition after controlling for the effects of age, gender, and the apolipoprotein E ε4 (APOEε4) genotype. Although serum TSH levels were not associated with global cerebral Aβ deposition, they had a significant negative association with glucose metabolism in the precuneus/posterior cingulate cortex after controlling for age, gender, and the APOEε4 genotype. No other thyroid hormones exhibited relationships with either brain Aβ burden or glucose metabolism.

Conclusions

Even in a clinical euthyroid state, low serum fT4 and high serum TSH levels appear to be differentially associated with AD-specific brain changes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13195-017-0291-5) contains supplementary material, which is available to authorized users.

T3/TRs axis in hepatocellular carcinoma: new concepts for an old pair

Hepatocellular carcinoma (HCC) is a leading cause of cancer-related death worldwide, and its burden is expected to further increase in the next years. Chronic inflammation, induced by multiple viruses or metabolic alterations, and epigenetic and genetic modifications, cooperate in cancer development via a combination of common and distinct aetiology-specific pathways. In spite of the advances of classical therapies, the prognosis of this neoplasm has not considerably improved over the past few years. The advent of targeted therapies and the approval of the systemic treatment of advanced HCC with the kinase inhibitor sorafenib have provided some hope for the future. However, the benefits obtained from this treatment are still disappointing, as it extends the median life expectancy of patients by only few months. It is thus mandatory to find alternative effective treatments. Although the role played by thyroid hormones (THs) and their nuclear receptors (TRs) in human cancer is still unclear, mounting evidence indicates that they behave as oncosuppressors in HCC. However, the molecular mechanisms by which they exert this effect and the consequence of their activation following ligand binding on HCC progression remain elusive. In this review, we re-evaluate the existing evidence of the role of TH/TRs in HCC development; we will also discuss how TR alterations could affect fundamental biological processes, such as hepatocyte proliferation and differentiation, and consequently HCC progression. Finally, we will discuss if and how TRs can be foreseen as therapeutic targets in HCC and whether selective TR modulation by TH analogues may hold promise for HCC treatment.

Thyroid-Stimulating Hormone and Mild Cognitive Impairment: Results of the Heinz Nixdorf Recall Study

BACKGROUND

Although some studies reported on the association of serum thyroid-stimulating hormone (TSH) concentration and cognition, only one population-based study investigated the association of TSH concentration and mild cognitive impairment (MCI).

OBJECTIVE

To investigate the gender-specific association of low- and high-normal TSH concentrations with MCI in euthyroid participants.

METHODS

Analysis sample 1 included 2,563 euthyroid participants (aged 50-80 years) from the second examination of the population-based Heinz Nixdorf Recall study. Gender-specific TSH quintiles (Q1 low, Q2-Q4 middle, Q5 high TSH concentration) were determined and group comparisons of age- and education-adjusted mean scores were performed for all cognitive subtests. Analysis sample 2 included 378 participants with MCI and 931 cognitively normal participants. MCI was diagnosed according to previously published MCI criteria. Multivariate logistic regression models were performed using TSH quintiles (Q2-Q4 as reference) to assess the association of low- and high-normal TSH concentration with MCI. Models were performed unadjusted and adjusted for sociodemographic and cardiovascular risk factors.

RESULTS

Group comparisons showed significant differences only in the immediate recall of the verbal memory task in women. Only women showed a strong association of high-normal TSH concentration with MCI (unadjusted: odds ratio 2.09, 95% confidence interval 1.29-3.37, full adjusted: 1.86, 1.06-3.27). There was no association with low-normal TSH concentration in women and no association of either low- or high-normal TSH concentration with MCI in men.

CONCLUSIONS

These results suggest that women with high-normal TSH concentration might be at higher risk of cognitive decline. This needs to be confirmed in the longitudinal analysis.

Local hypothyroidism favors the progression of preneoplastic lesions to hepatocellular carcinoma in rats

Thyroid hormone receptors (TRs) are ligand-dependent transcription factors that mediate most of the effects elicited by the thyroid hormone, 3,5,3'-L-triiodothyronine (T3). TRs have been implicated in tumorigenesis, although it is unclear whether they act as oncogenes or tumor suppressors, and at which stage of tumorigenesis their dysregulation occurs. Using the resistant-hepatocyte rat model (R-H model), we found down-regulation of TRβ1 and TRα1 and their target genes in early preneoplastic lesions and hepatocellular carcinoma (HCCs), suggesting that a hypothyroid status favors the onset and progression of preneoplastic lesions to HCC. Notably, TRβ1 and, to a lesser extent, TRα1 down-regulation was observed only in preneoplastic lesions positive for the progenitor cell marker, cytokeratin-19 (Krt-19) and characterized by a higher proliferative activity, compared to the Krt-19 negative ones. TRβ1 down-regulation was observed also in the vast majority of the analyzed human HCCs, compared to the matched peritumorous liver or to normal liver. Hyperthyroidism induced by T3 treatment caused up-regulation of TRβ1 and of its target genes in Krt-19(+) preneoplastic rat lesions and was associated with nodule regression. In HCC, TRβ1 down-regulation was not the result of hypermethylation of its promoter, but was associated with an increased expression of TRβ1-targeting microRNAs ([miR]-27a, -181a, and -204). An inverse correlation between TRβ1 and miR-181a was also found in human cirrhotic peritumoral tissue, compared to normal liver.

CONCLUSION

Down-regulation of TRs, especially TRβ1, is an early and relevant event in liver cancer development and is species and etiology independent. The results also suggest that a hypothyroid status of preneoplastic lesions may contribute to their progression to HCC and that the reversion of this condition may represent a possible therapeutic goal to interfere with the development of this tumor.

Regulatory feedback loop between T3 and microRNAs in renal cancer

microRNAs, short non-coding RNAs, influence key physiological processes, including hormonal regulation, by affecting the expression of genes. In this study we hypothesised that the expression of microRNAs targeting thyroid hormone pathway genes may be in turn regulated by thyroid hormone signalling. It is known that the expression of DIO1, a gene contributing to triiodothyronine (T3) signalling, is regulated by miR-224. Thus, we analysed mutual regulation between triiodothyronine pathway and miR-224/miR-452/GABRE cluster. Firstly, we found that miR-452 directly regulates the expression of thyroid hormone receptor TRβ1 in renal cancer cells. In turn, the expression of miR-224/452/GABRE cluster and other microRNAs targeting TRβ1 was influenced by T3 treatment and/or TR silencing. miR-452 expression correlated with intracellular T3 concentrations in renal tumours. In conclusion, we propose a new mechanism of feedback regulation, by which in renal cancer microRNAs regulate the expression of T3 pathway genes, while T3 in turn regulates expression of microRNAs.

Reduced cerebrospinal fluid level of thyroxine in patients with Alzheimer's disease

BACKGROUND

Little is known of the association between thyroid hormones in the central nervous system and Alzheimer's disease (AD). We determined thyroid hormone levels in serum and cerebrospinal fluid (CSF) in a well-defined homogeneous mono-center population.

METHODS

Fifty-nine consecutive patients under primary evaluation for cognitive impairment were recruited. The participants included patients with AD or mild cognitive impairment (MCI) diagnosed with AD upon follow-up (n=31), patients with stable MCI (SMCI, n=13), patients with other dementias (n=15), and healthy controls (n=19). Thyroid hormones in serum and CSF and AD biomarkers in CSF were analyzed using established immunochemical assays. Cognitive impairment was estimated using mini-mental state examination (MMSE).

RESULTS

Serum levels of free and total thyroxine (T4) and triiodothyronine (T3) were similar in all groups whereas a marginal increase in serum thyroid-stimulating hormone (TSH) level was observed in the AD patients. The CSF level of total T4 was decreased in patients with AD and other dementias compared to SMCI (both P=0.01) and healthy controls (both P=0.001), whereas CSF levels of TSH and total T3 were unchanged. In the total study population, CSF total T4 level correlated positively with MMSE score (r=0.26, P<0.05) and negatively with CSF total-tau (T-Tau) level (r=-0.23, P<0.05).

CONCLUSION

Patients with AD as well as other dementias had signs of mild brain hypothyroidism, which could only to a small extent be detected in serum values.

Thyroid hormone regulation of APP (β-amyloid precursor protein) gene expression in brain and brain cultured cells

We have previously shown that the thyroid hormone triiodothyronine negatively regulates the transcriptional activity of the β-amyloid precursor protein gene (APP) in cultured murine neuroblastoma cells, by a mechanism that involves binding of the nuclear thyroid hormone receptor (TR) to DNA sequences located within the first exon of the gene. In this report we present results showing that the thyroid hormones also repress the expression of APP in human neuroblastoma cells and in primary cultures of rat neurons. In addition, and in agreement with the results obtained in cultured cells, APP messenger RNA and protein levels are significantly higher in the brain of hypothyroid rats and mice, and also in Alzheimer-related brain regions dissected from KO mice lacking TRs. These results show that binding of the thyroid hormones to their nuclear receptors mediate their repressive effect on APP gene expression in vivo.

Thyroid hormone suppression of β-amyloid precursor protein gene expression in the brain involves multiple epigenetic regulatory events

Thyroid hormone (T3) suppresses cerebral gene expression of the β-amyloid precursor protein (APP), an integral membrane protein that plays a key role in the onset and progression of Alzheimer's disease. However, the mechanisms by which T3 signaling pathways inhibit APP gene transcription in the brain remain unclear. By carrying out chromatin immunoprecipitation with neuroblastoma cells and primary rat brain tissue, we show for the first time that thyroid hormone receptors (TRs) directly bind at the APP gene in vivo at a promoter region containing a negative T3-response element. We further show that T3 treatment decreases both histone H3 acetylation and histone H3 lysine 4 methylation at the APP promoter and that chemical inhibitors of histone deacetylases and histone lysine demethylase abrogate T3-dependent APP silencing. Our findings thus suggest that TRs actively facilitate T3-dependent silencing of APP gene expression via the recruitment of distinct histone modifying enzymes associated with transcriptional repression.

Thyroid hormone receptor beta (THRB) is a major target gene for microRNAs deregulated in papillary thyroid carcinoma (PTC)

CONTEXT

Loss of the thyroid hormone receptor is common in tumors. In mouse models, a truncated THRB gene leads to thyroid cancer. Previously, we observed up-regulation of the expression of eight microRNAs (miRs) in papillary thyroid carcinoma (PTC) tumors.

OBJECTIVE

Our objective was to determine whether THRB might be inhibited by miRs up-regulated in PTC.

DESIGN

The potential binding of miR to the 3'-untranslated region of THRB was analyzed in silico. Direct inhibition by miRs binding to the cloned 3'-untranslated region of THRB was evaluated using luciferase assays. Inhibition of endogenous THRB and its target genes (DIO1 and APP) was examined in cell lines transfected by pre-miRs. The impact on thyroid hormone response element (TRE) was evaluated in promoter assays. Correlations between the expression of THRB and miRs was evaluated in 13 PTC tumor/normal tissue pairs.

RESULTS

THRB contains binding sites for the top seven miRs up-regulated in PTC (P = 0.0000002). Direct interaction with THRB was shown for miR-21 and miR-146a. We observed lower levels of THRB transcripts in cell lines transfected with miR-21, -146a, and -221 (down-regulation of 37-48%; P < 0.0001), but not with miR-181a. THRB protein was suppressed down to 10-28% by each of four miRs. Concomitant expression of DIO1 and APP was affected (down-regulation of 32-66%, P < 0.0034 and up-regulation of 48-57%, P < 0.0002, respectively). All four miRs affected TRE activity in promoter assays. Down-regulation of luciferase occurred after transfection with pTRE-TK-Luc construct and each of four miRs. The analysis of tumor/normal tissue pairs revealed down-regulation of THRB in 11 of 13 pairs (1.3- to 9.1-fold), and up-regulation of miR-21, -146a, -181a, and -221 in almost all pairs.

CONCLUSIONS

MiRs up-regulated in PTC tumors directly inhibit the expression of THRB, an important tumor suppressor gene.

Thyroid hormones are associated with poorer cognition in mild cognitive impairment

BACKGROUND

Alterations in interrelated endocrine axes may be related to the pathogenesis of mild cognitive impairment (MCI) and dementia.

METHODS

Salivary cortisol before and after a 0.5-mg dexamethasone test, and serum levels of thyroid-stimulating hormone, total thyroxine (T(4)), free T(4), total triiodothyronine (TT(3)), estradiol, testosterone and insulin-like growth factor 1 were measured in 43 MCI cases and 26 healthy controls. All participants underwent a comprehensive neuropsychological test battery covering the cognitive domains of speed/attention, memory, visuospatial functions, language and executive functions.

RESULTS

The MCI group did not differ in basal levels of endocrine markers compared to controls. Among those with MCI, TT(3) levels were inversely associated with cognitive performance across all domains. After stratifying MCI cases according to TT(3) levels, those with relatively high TT(3) levels showed impairment in memory as well as in visuospatial and executive functions. Those with TT(3) levels at or below the lower boundary of the normal range performed comparably to healthy controls. Other endocrine markers were not related to cognitive performance.

CONCLUSIONS

Among those with MCI, TT(3) was associated with a neuropsychological profile typical of prodromal Alzheimer's disease. While the mechanisms remain unclear, optimal levels of thyroid hormone under a compromising condition such as MCI and related neuropathology need reconsideration.

Thyroid medication use and subsequent development of dementia of the Alzheimer type

Associations between medication use and the development of Alzheimer disease have been investigated since the late 1900s. Thyroid hormone supplementation is rarely a studied medication class in this area of research. We examined data from participants enrolled in longitudinal studies at the Washington University Alzheimer's Disease Research Center for associations between thyroid disease, thyroid hormone supplementation therapy, and subsequent development of dementia of the Alzheimer type (DAT). Data collected between April 1992 and June 2008 from 499 participants, 184 men and 315 women, were analyzed. Mean age was 76.9 years (SD = 9.2). At baseline, 61 participants reported thyroid medication use and 87 were identified as having a history of thyroid dysfunction. These participants progressed to a DAT diagnosis more rapidly than individuals not taking thyroid medication (hazard ratios [HR]: 1.67, 95% CI: 0.99-2.78, P = .054). Although an interesting trend was seen, baseline thyroid disease was not significantly (P = .093) associated with time to DAT diagnosis. Our findings suggest that utilization of thyroid medication may be associated with the development of DAT.

Thyroid function and the risk of Alzheimer disease: the Framingham Study

BACKGROUND

Clinical hypothyroidism and hyperthyroidism are recognized causes of reversible dementia, but previous studies relating thyrotropin levels to cognitive performance in clinically euthyroid persons have yielded inconsistent results.

METHODS

We related serum thyrotropin concentrations measured at baseline (March 1977-November 1979) to the risk of Alzheimer disease (AD) in 1864 cognitively intact, clinically euthyroid Framingham original cohort participants (mean age, 71 years; 59% women). Sex-specific Cox proportional hazards models were constructed using tertiles of thyrotropin concentration (tertile 2 as the referent) and adjusting for age, apolipoprotein E epsilon4 allele status, educational level, plasma homocysteine level, current smoking, body mass index, prevalent stroke, and atrial fibrillation.

RESULTS

During a mean follow-up of 12.7 years (range, 1-25 years), 209 participants (142 women) developed AD. Women in the lowest (<1.0 mIU/L) and highest (>2.1 mIU/L) tertiles of serum thyrotropin concentration were at increased risk for AD (multivariate-adjusted hazard ratio, 2.39 [95% confidence interval, 1.47-3.87] [P < .001] and 2.15 [95% confidence interval, 1.31-3.52] [P = .003], respectively) compared with those in the middle tertile. Thyrotropin levels were not related to AD risk in men. Analyses excluding individuals receiving thyroid supplementation did not significantly alter these relationships. In analyses limited to participants with serum thyrotropin levels of 0.1 to 10.0 mIU/L, the U-shaped relationship between thyrotropin level and AD risk was maintained in women but not when analyses were limited to those with thyrotropin levels of 0.5 to 5.0 mIU/L.

CONCLUSION

Low and high thyrotropin levels were associated with an increased risk of incident AD in women but not in men.