American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

Iodine Deficiency Increases Fat Contribution to Energy Expenditure in Male Mice

More than a billion people worldwide are at risk of iodine deficiency (ID), with well-known consequences for development of the central nervous system. Furthermore, ID has also been associated with dyslipidemia and obesity in humans. To further understand the metabolic consequences of ID, here we kept 8-week-old C57/Bl6 mice at thermoneutrality (~28°C) while feeding them on a low iodine diet (LID). When compared with mice kept on control diet (LID + 0.71 μg/g iodine), the LID mice exhibited marked reduction in T4 and elevated plasma TSH, without changes in plasma T3 levels. LID mice grew normally, and had normal oxygen consumption, ambulatory activity, and heart expression of T3-responsive gene, confirming systemic euthyroidism. However, LID mice exhibited ~5% lower respiratory quotient (RQ), which reflected a ~2.3-fold higher contribution of fat to energy expenditure. LID mice also presented increased circulating levels of nonesterified fatty acids, ~60% smaller fat depots, and increased hepatic glycogen content, all indicative of accelerated lipolysis. LID mice responded much less to forced mobilization of energy substrates (50% food restriction for 3 days or starvation during 36 hours) because of limited size of the adipose depots. A 4-day treatment with T4 restored plasma T4 and TSH levels in LID mice and normalized RQ. We conclude that ID accelerates lipolysis and fatty acid oxidation, without affecting systemic thyroid hormone signaling. It is conceivable that the elevated plasma TSH levels trigger these changes by directly activating lipolysis in the adipose tissues.

Early life variations in temperature exposure affect the epigenetic regulation of the paraventricular nucleus in female rat pups

Early life maternal care received has a profound effect on later-life behaviour in adult offspring, and previous studies have suggested epigenetic mechanisms are involved. Changes in thyroid hormone receptor signalling may be related to differences in maternal care received and DNA methylation modifications. We investigated the effects of variations in temperature exposure (a proxy of maternal contact) and licking-like tactile stimulation on these processes in week-old female rat pups. We assessed thyroid hormone receptor signalling by measuring circulating triiodothyronine and transcript abundance of thyroid hormone receptors and the thyroid hormone-responsive genes DNA methyltransferase 3a and oxytocin in the paraventricular nucleus of the hypothalamus. DNA methylation of the oxytocin promoter was assessed in relation to changes in thyroid hormone receptor binding. Repeated room temperature exposure was associated with a decrease in thyroid hormone receptor signalling measures relative to nest temperature exposure, while acute room temperature exposure was associated with an increase. Repeated room temperature exposure also increased thyroid hormone receptor binding and DNA methylation at the oxytocin promoter. These findings suggest that repeated room temperature exposure may affect DNA methylation levels as a consequence of alterations in thyroid hormone receptor signalling.

Thyroid redox imbalance in adult Wistar rats that were exposed to nicotine during breastfeeding

Maternal nicotine exposure causes several consequences in offspring phenotype, such as obesity and thyroid dysfunctions. Nicotine exposure can increase oxidative stress levels, which could lead to thyroid dysfunction. However, the mechanism by which nicotine exposure during breastfeeding leads to thyroid gland dysfunction remains elusive. We aimed to investigate the long-term effects of maternal nicotine exposure on redox homeostasis in thyroid gland, besides other essential steps for thyroid hormone synthesis in rats from both sexes. Lactating Wistar rats were implanted with osmotic minipumps releasing nicotine (NIC, 6 mg/kg/day) or saline (control) from postnatal day 2 to 16. Offspring were analyzed at 180-day-old. NIC males showed lower plasma TSH, T₃ and T₄ while NIC females had higher T₃ and T₄. In thyroid, NIC males had higher sodium-iodide symporter protein expression, whereas NIC females had higher thyroid-stimulating hormone receptor (TSHr) and thyroperoxidase (TPO) protein expression. TPO activity was lower in NIC males. Hydrogen peroxide generation was decreased in NIC males. Activities of superoxide dismutase, catalase and glutathione peroxidase were compromised in NIC animals from both sexes. 4-Hydroxynonenal was higher only in NIC females, while thiol was not affected in NIC animals from both sexes. NIC offspring also had altered expression of sex steroid receptors in thyroid gland. Both sexes showed similar thyroid morphology, with lower follicle and colloid size. Thyroid from female offspring exposed to nicotine during breastfeeding developed oxidative stress, while the male gland seemed to be protected from redox damage. Thyroid dysfunctions seem to be associated with redox imbalance in a sex-dependent manner.

Aging Alters Phenotypic Traits of Thyroid Dysfunction in Male Mice With Divergent Effects on Complex Systems but Preserved Thyroid Hormone Action in Target Organs

Clinical manifestation of hyperthyroidism and hypothyroidism vary with age, with an attenuated, oligosymptomatic presentation of thyroid dysfunction (TD) in older patients. We asked, whether in rodents TD phenotypes are influenced by age and whether this involves changes in systemic and/or organ thyroid hormone (TH) signaling. Chronic hyper- or hypothyroidism was induced in male mice at different life stages (5, 12, and 20 months). TH excess resulted in pronounced age-specific body weight changes (increase in youngest and decrease in old mice), neither explained by changes in food intake (similar increase at all ages), nor by thermogenic gene expression in brown adipose tissue (BAT) or TH serum concentrations. Relative increase in body temperature and activity were more pronounced in old compared to young hyperthyroid mice. An attenuated hypothyroid state was found in old mice for locomotor activity and in heart and BAT on functional (less bradycardia) and gene expression level (heart and BAT). In contrast, decrease in body weight was pronounced in old hypothyroid mice. Thus, age has divergent impact on features of TD in mice, whereby effects on highly complex systems, such as energy homeostasis are not proportional to serum TH state, in contrast to organ-specific responses in heart and BAT.

TAK1-AMPK Pathway in Macrophages Regulates Hypothyroid Atherosclerosis

PURPOSE

Hypothyroidism (HT) is associated with accelerated atherosclerosis (AS). The efficacy of traditional strategies of hypothyroid AS remains controversial. Here, we aimed to deepen the understanding of the HT-induced acceleration of AS, to decrease the residual risk of coronary artery disease (CAD) and to find a new therapeutic target.

METHODS

We collected peripheral venous blood samples from 20 patients and divided them into 4 groups, namely, the normal group, the HT group, the CAD group and the HT + CAD group. Then we performed mRNA microarray analysis and bioinformatics analysis to screen the differentially expressed genes and pathways, and we also conducted validations on ApoE knockout mice models and Raw264.7 cell models.

RESULTS

In short, (1) in the analysis between the CAD group and the HT + CAD group, we found a total of 1218 differentially expressed genes, 11 upregulated pathways and 40 downregulated pathways. (2) We validated that patients with HT and CAD had a significantly decreased expression of MAP3K7 (encoding transforming growth factor-β-activated kinase 1, TAK1) gene than normal subjects. (3) In animal and cell experiments, we found the decreased expression of TAK1 and the reduced phosphorylation of AMP-activated protein kinase (AMPK) under the hypothyroid and atherosclerotic condition. (4) Changes in the expressions of TAK1 may affect the progression of AS.

CONCLUSION

Taken together, these data suggest that the accelerated AS in hypothyroid patients may be due to the suppression of TAK1-AMPK pathway in macrophages. This new finding may become a novel therapeutic target in hypothyroid AS.

Development of an In Vitro Human Thyroid Microtissue Model for Chemical Screening

Thyroid hormones (TH) are essential for regulating a number of diverse physiological processes required for normal growth, development, and metabolism. The US EPA Endocrine Disruptor Screening Program (EDSP) has identified several molecular thyroid targets relevant to hormone synthesis dynamics that have been adapted to high-throughput screening (HTS) assays to rapidly evaluate the ToxCast/Tox21 chemical inventories for potential thyroid disrupting chemicals (TDCs). The uncertainty surrounding the specificity of active chemicals identified in these screens and the relevance to phenotypic effects on in vivo human TH synthesis are notable data gaps for hazard identification of TDCs. The objective of this study was to develop a medium-throughput organotypic screening assay comprised of reconstructed human thyroid microtissues to quantitatively evaluate the disruptive effects of chemicals on TH production and secretion. Primary human thyroid cells procured from qualified euthyroid donors were analyzed for retention of NK2 homeobox 1 (NKX2-1), Keratin 7 (KRT7), and Thyroglobulin (TG) protein expression by high-content image analysis to verify enrichment of follicular epithelial cells. A direct comparison of 2-dimensional (2D) and 3-dimensional (3D) 96-well culture formats was employed to characterize the morphology, differential gene expression, TG production, and TH synthesis over the course of 20 days. The results indicate that modeling human thyroid cells in the 3D format was sufficient to restore TH synthesis not observed in the 2D culture format. Inhibition of TH synthesis in an optimized 3D culture format was demonstrated with reference chemicals for key molecular targets within the thyroid gland. Implementation of the assay may prove useful for interpreting phenotypic effects of candidate TDCs identified by HTS efforts currently underway in the EDSP.

Auto-Regulation of the Thyroid Gland Beyond Classical Pathways

This mini-review asks how self-regulation of the thyroid gland is realized at the cellular and molecular levels by canonical and non-canonical means. Canonical pathways of thyroid regulation comprise thyroid stimulating hormone-triggered receptor signaling. As part of non-canonical regulation, we hypothesized an interplay between protease-mediated thyroglobulin processing and thyroid hormone release into the circulation by means of thyroid hormone transporters like Mct8. We proposed a sensing mechanism by different thyroid hormone transporters, present in specific subcellular locations of thyroid epithelial cells, selectively monitoring individual steps of thyroglobulin processing, and thus, the cellular thyroid hormone status. Indeed, we found that proteases and thyroid hormone transporters are functionally inter-connected, however, in a counter-intuitive manner fostering self-thyrotoxicity in particular in Mct8- and/or Mct10-deficient mice. Furthermore, the possible role of the G protein-coupled receptor Taar1 is discussed, because we detected Taar1 at cilia of the apical plasma membrane of thyrocytes in vitro and in situ. Eventually, through pheno-typing Taar1-deficient mice, we identified a co-regulatory role of Taar1 and the thyroid stimulating hormone receptors. Recently, we showed that inhibition of thyroglobulin-processing enzymes results in disappearance of cilia from the apical pole of thyrocytes, while Taar1 is re-located to the endoplasmic reticulum. This pathway features a connection between thyrotropin-stimulated secretion of proteases into the thyroid follicle lumen and substrate-mediated self-assisted control of initially peri-cellular thyroglobulin processing, before its reinternalization by endocytosis, followed by extensive endo-lysosomal liberation of thyroid hormones, which are then released from thyroid follicles by means of thyroid hormone transporters.

Relevant dose of the environmental contaminant, tributyltin, promotes histomorphological changes in the thyroid gland of male rats

Organotin compounds, such as tributyltin (TBT), are common environmental contaminants and suspected endocrine-disrupting chemicals. Tributyltin is found in antifouling paints, widely used in ships and other vessels. The present study evaluated whether a 15-day treatment with TBT at a dose of 100 ng/kg/day could induce histomorphological changes in the thyroid gland of rats. TBT promoted relevant alterations in the thyroid architecture, being the most relevant histological findings the presence of increased number of small-size follicles in the treated group. In qualitative analyses, colloid vacuolization, papillary budging structures, cystic degeneration and chronic thyroiditis, were observed. Moreover, histomorphometric analysis showed statistically significant changes in the follicular architecture of TBT-treated rats, mainly a decrease in the follicle area (colloid) and an increased epithelial height that resulted in an increased epithelial height/colloid ratio. Augmented collagen deposition was also seen in the thyroids of treated groups. In immunohistochemical (IHC) analyses, the localization of NIS protein was described and a significant increased proliferation index (evaluated by Ki67 positive cells) in the treated group was reported. As an indirect measurement of oxidative stress, mitochondrial protein SDHA was also analyzed by IHC analysis. Although the cytoplasmic expression of SDHA was observed in both groups, the staining intensity score was higher in TBT-treated group. Our results suggest that besides causing histomorphological changes, environmental relevant dose of TBT treatment can also induce oxidative alterations.

Demonstration of reciprocal diurnal variation in human serum T3 and rT3 concentration demonstrated by mass spectrometric analysis and establishment of thyroid hormone reference intervals

BACKGROUND

There has been a wide range of reference intervals proposed in previous literature for thyroid hormones due to large between-assay variability of immunoassays, as well as lack of correction for collection time. We provided the diurnal reference intervals for five thyroid hormones, namely total thyroxine (TT4), total triiodothyronine (TT3), free thyroxine (FT4), free triiodothyronine (FT3), and reverse T3 (rT3), measured in serum samples of healthy participants using a liquid chromatography/tandem mass spectrometry (LC-MS/MS) method.

METHODS

Couplet serum samples (a.m. and p.m.) were collected from 110 healthy females and 49 healthy males. Healthy volunteers were recruited from four participating centers between 2016 and 2018. Measurements of thyroid hormones were obtained by LC-MS/MS analysis.

RESULTS

Our study revealed significant uptrend in AM to PM FT4 (p < 0.0001) samples, downtrend in AM to PM TT3 (p = 0.0004) and FT3 samples (p < 0.0001), and AM to PM uptrend in rT3 samples (p < 0.0001). No difference was observed for TT4 between AM and PM. No significant sex differences were seen for any of the five thyroid hormones.

CONCLUSION

When diagnosing thyroid disorders, it is important to have accurate measurement of thyroid hormones, and to acknowledge the diurnal fluctuation found, especially for FT3. Our study highlights the importance of standardization of collection times and implementation of LC-MS/MS in thyroid hormone measurement.

Imbalanced testicular metabolism induced by thyroid disorders: New evidences from quantitative proteome

Thyroid dysfunctions, such as hypothyroidism and hyperthyroidism, are the second most prevalent endocrinopathies and are associated to reproductive disorders in men. Several genes are differentially modulated by thyroid hormones in testes and imbalances in thyroid hormone levels are also associated to alterations on sperm functionality. Imbalances on antioxidant defense mechanism and stress oxidative have been pointed out as the main factors for the impairments on male reproductive function. To clarify this issue, we investigated the expression and activity of antioxidant enzymes in testis, followed by their proteomic profile in attempt to characterize the mechanisms involved in the alterations induced by hypo- or hyperthyroidism in adult male rats. Hypothyroidism reduced the Gsr transcript expression and the activity of CAT and GSR enzymes, while the hyperthyroidism reduced the Gpx4 var2 transcript expression. Among 1082 identified proteins, 123 and 37 proteins were downregulated by hypothyroidism compared to euthyroid and hyperthyroid condition, respectively, being 36 proteins commonly reduced in both comparisons and one exclusively in hypo-hyperthyroidism comparison. A network containing 29 nodes and 68 edges was obtained in protein-protein interaction analysis and the functional enrichment analysis of differentially expressed proteins revealed significant alterations for several functions in hypo-euthyroid and hypo-hyperthyroid comparisons, such as ATP metabolic process, coenzyme binding, sperm part, peroxiredoxin activity, mitochondrial protein complex, intramolecular oxidoreductase activity, binding of sperm to zona pellucida, glutathione transferase activity, response to testosterone. Thus, there is a correlation between thyroid disorders and impaired antioxidant defense mechanism, resulting in reproductive dysfunctions, as infertility, mainly observed in hypothyroidism.

Unraveling the Molecular Basis for Successful Thyroid Hormone Replacement Therapy: The Need for New Thyroid Tissue- and Pathway-Specific Biomarkers

Thyroid function is conventionally assessed by measurement of thyroid-stimulating hormone (TSH) and free circulating thyroid hormones, which is in most cases sufficient for correct diagnosis and monitoring of treatment efficiency. However, several conditions exist, in which these parameters may be insufficient or even misleading. For instance, both, a TSH-secreting pituitary adenoma and a mutation of thyroid hormone receptor β present with high levels of TSH and circulating hormones, but the optimal treatment is substantially different. Likewise, changes in thyroid hormone receptor α signaling are not captured by routine assessment of thyroid status, as serum parameters are usually inconspicuous. Therefore, new biomarkers are urgently needed to improve the diagnostic management and monitor treatment efficiency for e. g., replacement therapy in hypothyroidism or thyroid hormone resistance. By comparing animal models to human data, the present minireview summarizes the status of this search for new tissue- and pathway-specific biomarkers of thyroid hormone action.

Basal Autophagy Deficiency Causes Thyroid Follicular Epithelial Cell Death in Mice

Autophagy is a catabolic process that involves the degradation of cellular components through the lysosomal machinery, relocating nutrients from unnecessary processes to more pivotal processes required for survival. It has been reported that systemic disruption of the Atg5 or Atg7 gene, a component of autophagy, is lethal and that its tissue-specific disruption causes tissue degeneration in several organs. However, the functional significance of autophagy in the thyroid glands remains unknown. Our preliminary data imply the possible involvement of dysfunctional autophagy in radiation-induced thyroid carcinogenesis. Therefore, we evaluated the effect of Atg5 gene knockout (KO) on thyroid morphology and function. To this end, Atg5flox/flox mice were crossed with TPO-Cre mice, yielding the thyroid follicular epithelial cell (thyrocyte)‒specific ATG5-deficient mice (Atg5thyr-KO/KO). Atg5 gene KO was confirmed by a lack of ATG5 expression, and disruption of autophagy was demonstrated by a decrease in microtubule-associated protein 1 light chain 3-II puncta and an increase in p62. Atg5thyr-KO/KO mice were born normally, and thyroid morphology, thyroid weights, and serum T4 and TSH levels were almost normal at 4 months. However, at 8 and 12 months, a decrease in the number of thyrocytes and an increase in TUNEL+-thyrocytes were observed in Atg5thyr-KO/KO mice even though thyroid function was still normal. The number of irregularly shaped (gourd-shaped) follicles was also increased. Excess oxidative stress was indicated by increased 8-hydroxy-2'-deoxyguanosine and 53BP1 foci in Atg5thyr-KO/KO mice. These data demonstrate that thyrocytes gradually undergo degradation/cell death in the absence of basal levels of autophagy, indicating that autophagy is critical for the quality control of thyrocytes.

Nonalcoholic Fatty Liver Disease and Hypercholesterolemia: Roles of Thyroid Hormones, Metabolites, and Agonists

Background: Thyroid hormones (THs) exert a strong influence on mammalian lipid metabolism at the systemic and hepatic levels by virtue of their roles in regulating circulating lipoprotein, triglyceride (TAG), and cholesterol levels, as well as hepatic TAG storage and metabolism. These effects are mediated by intricate sensing and feedback systems that function at the physiological, metabolic, molecular, and transcriptional levels in the liver. Dysfunction in the pathways involved in lipid metabolism disrupts hepatic lipid homeostasis and contributes to the pathogenesis of metabolic diseases, such as nonalcoholic fatty liver disease (NAFLD) and hypercholesterolemia. There has been strong interest in understanding and employing THs, TH metabolites, and TH mimetics as lipid-modifying drugs. Summary: THs regulate many processes involved in hepatic TAG and cholesterol metabolism to decrease serum cholesterol and intrahepatic lipid content. TH receptor β analogs designed to have less side effects than the natural hormone are currently being tested in phase II clinical studies for NAFLD and hypercholesterolemia. The TH metabolites, 3,5-diiodo-l-thyronine (T2) and T1AM (3-iodothyronamine), have different beneficial effects on lipid metabolism compared with triiodothyronine (T3), although their clinical application is still under investigation. Also, prodrugs and glucagon/T3 conjugates have been developed that direct TH to the liver. Conclusions: TH-based therapies show clinical promise for the treatment of NAFLD and hypercholesterolemia. Strategies for limiting side effects of TH are being developed and may enable TH metabolites and analogs to have specific effects in the liver for treatments of these conditions. These liver-specific effects and potential suppression of the hypothalamic/pituitary/thyroid axis raise the issue of monitoring liver-specific markers of TH action to assess clinical efficacy and dosing of these compounds.

Paradigms of Dynamic Control of Thyroid Hormone Signaling

Thyroid hormone (TH) molecules enter cells via membrane transporters and, depending on the cell type, can be activated (i.e., T4 to T3 conversion) or inactivated (i.e., T3 to 3,3'-diiodo-l-thyronine or T4 to reverse T3 conversion). These reactions are catalyzed by the deiodinases. The biologically active hormone, T3, eventually binds to intracellular TH receptors (TRs), TRα and TRβ, and initiate TH signaling, that is, regulation of target genes and other metabolic pathways. At least three families of transmembrane transporters, MCT, OATP, and LAT, facilitate the entry of TH into cells, which follow the gradient of free hormone between the extracellular fluid and the cytoplasm. Inactivation or marked downregulation of TH transporters can dampen TH signaling. At the same time, dynamic modifications in the expression or activity of TRs and transcriptional coregulators can affect positively or negatively the intensity of TH signaling. However, the deiodinases are the element that provides greatest amplitude in dynamic control of TH signaling. Cells that express the activating deiodinase DIO2 can rapidly enhance TH signaling due to intracellular buildup of T3. In contrast, TH signaling is dampened in cells that express the inactivating deiodinase DIO3. This explains how THs can regulate pathways in development, metabolism, and growth, despite rather stable levels in the circulation. As a consequence, TH signaling is unique for each cell (tissue or organ), depending on circulating TH levels and on the exclusive blend of transporters, deiodinases, and TRs present in each cell. In this review we explore the key mechanisms underlying customization of TH signaling during development, in health and in disease states.

The Foxo1-Inducible Transcriptional Repressor Zfp125 Causes Hepatic Steatosis and Hypercholesterolemia

Liver-specific disruption of the type 2 deiodinase gene (Alb-D2KO) results in resistance to both diet-induced obesity and liver steatosis in mice. Here, we report that this is explained by an ∼60% reduction in liver zinc-finger protein-125 (Zfp125) expression. Zfp125 is a Foxo1-inducible transcriptional repressor that causes lipid accumulation in the AML12 mouse hepatic cell line and liver steatosis in mice by reducing liver secretion of triglycerides and hepatocyte efflux of cholesterol. Zfp125 acts by repressing 18 genes involved in lipoprotein structure, lipid binding, and transport. The ApoE promoter contains a functional Zfp125-binding element that is also present in 17 other lipid-related genes repressed by Zfp125. While liver-specific knockdown of Zfp125 causes an "Alb-D2KO-like" metabolic phenotype, liver-specific normalization of Zfp125 expression in Alb-D2KO mice rescues the phenotype, restoring normal susceptibility to diet-induced obesity, liver steatosis, and hypercholesterolemia.

Decreased deiodinase activity after glucose load could lead to atherosclerosis in euthyroid women with polycystic ovary syndrome

OBJECTIVE

Glucose and lipid disturbances, as well as higher tendency to atherosclerosis, are observed in women with polycystic ovary syndrome (PCOS). Thyroid hormones action has long been recognized as an important determinant of glucose and lipid homeostasis. Some studies suggest that even in euthyroid subjects, thyroid function may affect atherosclerosis risk factors. The aim of this study was to evaluate the relationships between thyroid hormonal status and glucose and lipid profile before and after oral glucose tolerance test (OGTT) in PCOS women in comparison to the control group.

PATIENTS AND METHODS

The study group included 98 women-60 women with PCOS and 38 women matched for age and BMI as a control group. OGTT with estimation of plasma glucose and lipids, as well as serum insulin and thyroid hormones (TH) concentrations was performed. Activity of peripheral deiodinases at baseline (SPINA-GD1) and at the 120 min of OGTT (SPINA-GD2) was calculated according to the formula by Dietrich et al. as a measure of T4-T3 conversion efficiency. Delta GD was estimated as SPINA-GD1-SPINA-GD2, and delta fT3 was calculated as a difference between fT3 before and after OGTT.

RESULTS

We did not find differences in TH, SPINA-GDs, and plasma lipid concentrations between PCOS and control group before and after OGTT. Glucose load resulted in a decrease of level TSH, TC, TG, HDL-C, and LDL-C concentrations in women with PCOS, as well as in the control group (all p < 0.05). We found that GD (p = 0.01) and serum fT3 concentration (p = 0.0008) decreased during glucose load only in the PCOS group. We observed a positive relationship between delta fT3 and plasma TG concentration (r = 0.36, p = 0.004), delta GD and plasma TG concentration after glucose load (r = 0.34, p = 0.007), only in the PCOS group. We also found negative relationship between SPINA-GD2 and plasma TC concentration (r = -0.29, p = 0.02) after glucose load and positive relationship between delta GD and insulin at the 60 min of OGTT (r = 0.29, p = 0.02), only in the PCOS women.

CONCLUSIONS

These data showed insufficient conversion of fT4 to fT3, as well as a relationship of SPINA-GDs with insulin, TC and TG in PCOS women after glucose load. It may suggest that disturbances in deiodinase activity after glucose load might promote atherosclerosis in PCOS women.

Practical considerations for developmental thyroid toxicity assessments: What's working, what's not, and how can we do better?

Thyroid hormones (THs; T3 and T4) play a role in development of cardiovascular, reproductive, immune and nervous systems. Thus, interpretation of TH changes from rodent studies (during pregnancy, in fetuses, neonates, and adults) is critical in hazard characterization and risk assessment. A roundtable session at the 2017 Society of Toxicology (SOT) meeting brought together academic, industry and government scientists to share knowledge and different perspectives on technical and data interpretation issues. Data from a limited group of laboratories were compiled for technical discussions on TH measurements, including good practices for reliable serum TH data. Inter-laboratory historical control data, derived from immunoassays or mass spectrometry methods, revealed: 1) assay sensitivities vary within and across methodologies; 2) TH variability is similar across animal ages; 3) laboratories generally achieve sufficiently sensitive TH quantitation levels, although issues remain for lower levels of serum TH and TSH in fetuses and postnatal day 4 pups; thus, assay sensitivity is critical at these life stages. Best practices require detailed validation of rat serum TH measurements across ages to establish assay sensitivity and precision, and identify potential matrix effects. Finally, issues related to data interpretation for biological understanding and risk assessment were discussed, but their resolution remains elusive.

Myelin repair stimulated by CNS-selective thyroid hormone action

Oligodendrocyte processes wrap axons to form neuroprotective myelin sheaths, and damage to myelin in disorders, such as multiple sclerosis (MS), leads to neurodegeneration and disability. There are currently no approved treatments for MS that stimulate myelin repair. During development, thyroid hormone (TH) promotes myelination through enhancing oligodendrocyte differentiation; however, TH itself is unsuitable as a remyelination therapy due to adverse systemic effects. This problem is overcome with selective TH agonists, sobetirome and a CNS-selective prodrug of sobetirome called Sob-AM2. We show here that TH and sobetirome stimulated remyelination in standard gliotoxin models of demyelination. We then utilized a genetic mouse model of demyelination and remyelination, in which we employed motor function tests, histology, and MRI to demonstrate that chronic treatment with sobetirome or Sob-AM2 leads to significant improvement in both clinical signs and remyelination. In contrast, chronic treatment with TH in this model inhibited the endogenous myelin repair and exacerbated disease. These results support the clinical investigation of selective CNS-penetrating TH agonists, but not TH, for myelin repair.

ER stress contributes to high-fat diet-induced decrease of thyroglobulin and hypothyroidism

Recent studies revealed the emerging role of excess uptake of lipids in the development of hypothyroidism. However, the underlying mechanism is largely unknown. We investigated the effect of high-fat diet (HFD) on thyroid function and the role of endoplasmic reticulum (ER) in HFD-induced hypothyroidism. Male Sprague-Dawley rats were fed with HFD or control diet for 18 wk. HFD rats showed an impaired thyroid function, with decreased thyroglobulin (Tg) level. We found the ER stress was triggered in HFD rat thyroid glands and palmitate-treated thyrocytes. Luminal swelling of ER in thyroid epithelial cells of HFD rats was also observed. The rate of Tg degradation increased in palmitate-treated thyrocytes. In addition, applying 4-phenyl butyric acid to alleviate ER stress in HFD rats improved the decrease of Tg and thyroid function. Withdrawal of the HFD improved thyroid function . In conclusion, we demonstrate that ER stress mediates the HFD-induced hypothyroidism, probably by impairing the production of Tg, and attenuation of ER stress improves thyroid function. Our study provides the understanding of how HFD induces hypothyroidism.

Investigation of adrenal and thyroid gland dysfunction in dogs with ultrasonographic diagnosis of gallbladder mucocele formation

Gallbladder mucocele formation is an emerging disease in dogs characterized by increased secretion of condensed granules of gel-forming mucin by the gallbladder epithelium and formation of an abnormally thick mucus that can culminate in obstruction of the bile duct or rupture of the gallbladder. The disease is associated with a high morbidity and mortality and its pathogenesis is unknown. Affected dogs have a significantly increased likelihood of concurrent diagnosis of hyperadrenocorticism, hypothyroidism, and hyperlipidemia. Whether these endocrinopathies represent coincidental primary disease processes that exacerbate gallbladder mucocele formation in predisposed dogs or reflect a concurrent disruption of endocrine and lipid metabolism is unclear. In this study, we investigated a hypothesis that dogs with gallbladder mucocele formation would have a high prevalence of occult and atypical abnormalities in adrenal cortical and thyroid gland function that would suggest the presence of endocrine disruption and provide deeper insight into disease pathogenesis. We performed a case-control study of dogs with and without ultrasonographic diagnosis of gallbladder mucocele formation and profiled adrenal cortical function using a quantitative mass spectrometry-based assay of serum adrenal-origin steroids before and after administration of synthetic cosyntropin. We simultaneously profiled serum thyroid hormone concentrations and evaluated iodine sufficiency by measurement of urine iodine:creatinine ratios (UICR). The studies were complemented by histological examination of archival thyroid tissue and measurements of thyroid gland organic iodine from dogs with gallbladder mucocele formation and control dogs. Dogs with gallbladder mucocele formation demonstrated an exaggerated cortisol response to adrenal stimulation with cosyntropin. A prevalence of 10% of dogs with gallbladder mucocele formation met laboratory-based criteria for suspect or definitive diagnosis of hyperadrenocorticism. A significantly greater number of dogs with gallbladder mucocele formation had basal serum dehydroepiandrosterone (DHEAS) increases compared to control dogs. A high percentage of dogs with gallbladder mucocele formation (26%) met laboratory-based criteria for diagnosis of hypothyroidism, but lacked detection of anti-thyroglobulin antibodies. Dogs with gallbladder mucocele formation had significantly higher UICRs than control dogs. Examination of thyroid tissue from an unrelated group of dogs with gallbladder mucocele formation did not demonstrate histological evidence of thyroiditis or significant differences in content of organic iodine. These findings suggest that dogs with gallbladder mucocele formation have a greater capacity for cortisol synthesis and pinpoint DHEAS elevations as a potential clue to the underlying pathogenesis of the disease. A high prevalence of thyroid dysfunction with absent evidence for autoimmune thyroiditis suggest a disrupted thyroid hormone metabolism in dogs with gallbladder mucocele formation although an influence of non-thyroidal illness cannot be excluded. High UICR in dogs with gallbladder mucocele formation is of undetermined significance, but of interest for further study.

Evaluating the Hypothalamic-Pituitary-Thyroid (HPT) Axis in Mice

As the genome of experimental animals has become easier to manipulate, a number of mouse models have been developed to understand in vivo thyroid hormone action. A major site of thyroid hormone action is the HPT axis. While several methods are available that provide a detailed understanding of the HPT axis in mice, many authors choose to include only cursory data about this axis, which can lead to erroneous conclusions about in vivo thyroid hormone action. A standard protocol is proposed to evaluate the HPT axis in mice.

Thyroid hormones mediate metabolic rate and oxidative, anti-oxidative balance at different temperatures in Mongolian gerbils (Meriones unguiculatus)

Oxidative damage is a potential physiological cost of thermoregulation during seasonal adjustments to air temperature (T_a) in small mammals. Here, we hypothesized that T_a affects serum thyroid hormone levels and these hormones can mediate the changes in metabolic rate and oxidative damage. Mongolian gerbils (Meriones unguiculatus) were acclimated at different T_as (5 °C, 23 °C and 37 °C) for 3 weeks. Serum tri-iodothyronine (T3) levels increased at 5 °C but decreased at 37 °C compared to the control (23 °C). Protein carbonyls increased in liver at 37 °C compared with control, however, lipid damage (malonaldehyde, MDA) in both serum and liver was unrelated to T_a. After the effects of different T_as on thyroid hormone levels and oxidative damage markers were determined, we further investigate whether thyroid hormones mediated metabolic rate and oxidative damage. Another set of gerbils received 0.0036% L-thyroxin (hyperthyroid), 0.04% Methylimazol (hypothyroid) or water (control). Hypothyroid group showed a 34% reduction in resting metabolic rate (RMR) also 42% and 26% increases in MDA and liver protein carbonyl respectively, whereas hyperthyroid group had higher RMR, liver mass and superoxide dismutase (SOD) compared to control. Serum T3 or T3/T4 levels were correlated positively with RMR, liver mass, and SOD, but negatively with MDA and uncoupling protein 2 (UCP2). We concluded that high T_a induced hypothyroidism, decreased RMR and increased oxidative damage, whereas low T_a induced hyperthyroidism, increased RMR and unchanged oxidative damage. These data supported our hypothesis that thyroid hormones can be a cue to mediate metabolic rate and different aspects of oxidative and antioxidant activities at different T_as.

Type 2 deiodinase polymorphism causes ER stress and hypothyroidism in the brain

Levothyroxine (LT4) is a form of thyroid hormone used to treat hypothyroidism. In the brain, T4 is converted to the active form T3 by type 2 deiodinase (D2). Thus, it is intriguing that carriers of the Thr92Ala polymorphism in the D2 gene (DIO2) exhibit clinical improvement when liothyronine (LT3) is added to LT4 therapy. Here, we report that D2 is a cargo protein in ER Golgi intermediary compartment (ERGIC) vesicles, recycling between ER and Golgi. The Thr92-to-Ala substitution (Ala92-D2) caused ER stress and activated the unfolded protein response (UPR). Ala92-D2 accumulated in the trans-Golgi and generated less T3, which was restored by eliminating ER stress with the chemical chaperone 4-phenyl butyric acid (4-PBA). An Ala92-Dio2 polymorphism-carrying mouse exhibited UPR and hypothyroidism in distinct brain areas. The mouse refrained from physical activity, slept more, and required additional time to memorize objects. Enhancing T3 signaling in the brain with LT3 improved cognition, whereas restoring proteostasis with 4-PBA eliminated the Ala92-Dio2 phenotype. In contrast, primary hypothyroidism intensified the Ala92-Dio2 phenotype, with only partial response to LT4 therapy. Disruption of cellular proteostasis and reduced Ala92-D2 activity may explain the failure of LT4 therapy in carriers of Thr92Ala-DIO2.

Sex Dimorphic Responses of the Hypothalamus-Pituitary-Thyroid Axis to Maternal Separation and Palatable Diet

Neonatal stress contributes to the development of obesity and has long-lasting effects on elements of the hypothalamus-pituitary-thyroid (HPT) axis. Given the importance of thyroid hormones in metabolic regulation, we studied the effects of maternal separation and a high-fat/high-carbohydrate diet (HFC), offered from puberty or adulthood, on HPT axis activity of adult male and female Wistar rats. Pups were non-handled (NH) or maternally separated (MS) 3 h/day at postnatal days (Pd) 2-21. In a first experiment, at Pd60, rats had access to chow or an HFC diet (cookies, peanuts, chow) for 1 month. Male and female NH and MS rats that consumed the HFC diet increased their caloric intake, body weight, and serum insulin levels; fat weight increased in all groups except in MS males, and serum leptin concentration increased only in females. Mediobasal hypothalamus (MBH) Pomc expression increased in NH-HFC females and Npy decreased in NH-HFC males. MS males showed insulinemia and hypercortisolemia that was attenuated by the HFC diet. The HPT axis activity response to an HFC diet was sex-specific; expression of MBH thyrotropin-releasing hormone-degrading ectoenzyme (Trhde) increased in NH and MS males; serum TSH concentration decreased in NH males, and T4 increased in NH females. In a second experiment, rats were fed chow or an HFC diet from Pd30 or 60 until Pd160 and exposed to 1 h restraint before sacrifice. Regardless of neonatal stress, age of diet exposition, or sex, the HFC diet increased body and fat weight and serum leptin concentration; it induced insulinemia in males, but in females only in Pd30 rats. The HFC diet's capacity to curtail the hypothalamus-pituitary-adrenal axis response to restraint was impaired in MS males. In restrained rats, expression of Trh in the paraventricular nucleus of the hypothalamus, Dio2 and Trhde in MBH, and serum thyroid hormone concentration were altered differently depending on sex, age of diet exposition, and neonatal stress. In conclusion, metabolic alterations associated to an HFC-diet-induced obesity are affected by sex or time of exposition, while various parameters of the HPT axis activity are additionally altered by MS, pointing to the complex interplay that these developmental influences exert on HPT axis activity in adult rats.

Voluntary Exercise-Induced Activation of Thyroid Axis and Reduction of White Fat Depots Is Attenuated by Chronic Stress in a Sex Dimorphic Pattern in Adult Rats

The activity of the hypothalamus-pituitary-thyroid (HPT) axis is inhibited by energy deficit, by acute or chronic stress, but activated by cold exposure or exercise. Because stress curtails acute cold induced activation of HPT, we evaluated the effect of chronic stress on HPT axis response to voluntary exercise, a persistent energy-demanding situation. Adult male and female Wistar rats were exposed to restraint stress, 30 min/day for 2 weeks, or to isolation (Iso) [post-natal day [PND] 30-63]. Exercise was performed (7 p.m.-7 a.m.) in a running wheel, sedentary controls stayed in individual cages (Sed); at 7 a.m. they were housed with their cage mate or individually (Iso); food intake by the exercised group was measured day and night to pair-fed Sed. At sacrifice, hormones, mRNA levels and tissue weights were quantified. Control or restrained adult rats had access to running wheel daily for 2 weeks. Compared to C, exercise decreased white adipose tissue (WAT) mass in females and males, increased hypothalamic paraventricular nucleus (PVN)-Trh expression in males proportionally to exercise performed, and increased TSH and T4 serum concentration in females. These changes were not detected in restrained groups. Starting at PND 63 control (2/cage) and isolated (1/cage) rats either exercised on 10 alternated nights or were sedentary. In control male animals, compared to Sed rats, exercise did not decrease WAT mass, nor changed HPT axis activity, but increased Pomc and deiodinase 2 (Dio2) expression in mediobasal hypothalamus (MBH), adrenergic receptor β3 and uncoupling protein-1 in brown adipose tissue. In control female animals, exercise decreased WAT mass, increased Pomc, Dio2, and Trhde expression in MBH, and TSH serum concentration. Iso females had lower TSH and T4 serum concentration, Dio2 and Trhde expression in MBH than controls. The stress response was higher in isolated males than females, but in males it did not alter the effects of exercise, in contrast to isolated females that had a blunted response to exercise compared to controls. In conclusion, chronic stress interferes with metabolic effects produced by exercise, such as loss of WAT mass, coincident with dampening of HPT activity.

Disruptive Effect of Organotin on Thyroid Gland Function Might Contribute to Hypothyroidism

A considerable increase in endocrine abnormalities has been reported over the last few decades worldwide. A growing exposure to endocrine-disrupting chemicals (EDCs) can be one of the causes of endocrine disorders in populations, and these disorders are not only restricted to the metabolic hormone system but can also cause abnormal functions. Thyroid hormone (TH) disruption is defined as an abnormal change in TH production, transport, function, or metabolism, which results in some degree of impairment in body homeostasis. Many EDCs, including organotin compounds (OTCs), are environmental contaminants that are commonly found in antifouling paints used on ships and in several other industrial procedures. OTCs are obesogenic and can disrupt TH metabolism; however, abnormalities in thyroid function resulting from OTC exposure are less well understood. OTCs, one of the most prevalent EDCs that are encountered on a daily basis, modulate the thyroid axis. In most toxicology studies, it has been reported that OTCs might contribute to hypothyroidism.

Sustained Release T3 Therapy: Animal Models and Translational Applications

The standard of care to treat hypothyroidism is daily administration of levo-thyroxine (LT4). This is based on the understanding that deiodinases can restore production of T3 and compensate for the small amounts of T3 that are normally produced by the thyroid gland. However, pre-clinical and clinical evidence indicating that deiodinases fall short of restoring T3 production is accumulating, opening the possibility that liothyronine (LT3) might have a role in the treatment of some hypothyroid patients. LT3 tablets taken orally result in a substantial peak of circulating T3 that is dissipated during the next several hours, which is markedly distinct from the relative stability of T3 levels in normal individuals. Thus, the effort to developing new delivery strategies for LT3, including slow release tablets, liquid formulations, use of T3-related/hybrid molecules such as T3 sulfate, poly-zinc-T3 and glucagon-T3, nanoparticles containing T3, subcutaneous implant of T3-containing matrices, and stem cells for de novo development of the thyroid gland. This article reviews these strategies, their applicability in animal models and translatability to humans.

Metal Coordinated Poly-Zinc-Liothyronine Provides Stable Circulating Triiodothyronine Levels in Hypothyroid Rats

BACKGROUND

Liothyronine (LT3) has limited short-term clinical applications, all of which aim at suppressing thyrotropin (TSH) secretion. A more controversial application is chronic administration along with levothyroxine in the treatment of hypothyroidism. Long-term treatment with LT3 is complicated by its unique pharmacokinetics that result in a substantial triiodothyronine (T3) peak in the blood three to four hours after oral dosing. This is a significant problem, given that T3 levels in the blood are normally stable, varying by <10% throughout the day.

METHODS

A metal coordinated form of LT3 (Zn[T3][H₂O])n, known as poly-zinc-liothyronine (PZL), was synthesized and loaded into coated gelatin capsules for delivery to the duodenum where sustained release of T3 from PZL occurs. Male Wistar rats were made hypothyroid by feeding on a low iodine diet and water containing 0.05% methimazole for five to six weeks. Rats were given a capsule containing 24 μg/kg PZL or equimolar amounts of LT3. Blood samples were obtained multiple times from the tail vein during the first 16 hours, and processed for T3 and TSH serum levels. Some animals were treated daily for eight days, and blood samples were collected daily.

RESULTS

Rats given LT3 exhibited the expected serum T3 peak (about fivefold baseline) at 3.5 hours, followed by a rapid decline, with serum levels almost returning to baseline values by 16 hours. In contrast, serum T3 in PZL-treated rats exhibited about a 30% lower T3 peak at nine hours. Furthermore, the plateau time, that is, the time-span during which the serum T3 concentration is at least half of T3 peak, increased from 4.9 to 7.7 hours in LT3- versus PZL-treated rats, respectively. Serum TSH dropped in both groups, but PZL-treated rats exhibited a more gradual decrease, which was delayed by about four hours compared to LT3-treated rats. Chronic treatment with either LT3 or PZL restored growth, lowered serum cholesterol, and stimulated hepatic expression of the Dio1 mRNA and other T3-dependent markers in the central nervous system.

CONCLUSION

Capsules of PZL given orally restore T3-dependent biological effects while exhibiting a reduced and delayed serum T3 peak after dosing, thus providing a longer period of relatively stable serum T3 levels compared to capsules of LT3.

The environmental contaminant tributyltin leads to abnormalities in different levels of the hypothalamus-pituitary-thyroid axis in female rats

Tributyltin is a biocide used in nautical paints, aiming to reduce fouling of barnacles in ships. Despite the fact that many effects of TBT on marine species are known, studies in mammals have been limited, especially those evaluating its effect on the function of the hypothalamus-pituitary-thyroid (HPT) axis. The aim of this study was to investigate the effects of subchronic exposure to TBT on the HPT axis in female rats. Female Wistar rats received vehicle, TBT 200 ng kg^-1 BW d^-1 or 1000 ng kg^-1 BW d^-1 orally by gavage for 40 d. Hypothalamus, pituitary, thyroid, liver and blood samples were collected. TBT200 and TBT1000 thyroids showed vacuolated follicular cells, with follicular hypertrophy and hyperplasia. An increase in epithelial height and a decrease in the thyroid follicle and colloid area were observed in TBT1000 rats. Moreover, an increase in the epithelium/colloid area ratio was observed in both TBT groups. Lower TRH mRNA expression was observed in the hypothalami of TBT200 and TBT1000 rats. An increase in Dio1 mRNA levels was observed in the hypothalamus and thyroid in TBT1000 rats only. TSH serum levels were increased in TBT200 rats. In TBT1000 rats, there was a decrease in total T4 serum levels compared to control rats, whereas T3 serum levels did not show significant alterations. We conclude that TBT exposure can promote critical abnormalities in the HPT axis, including changes in TRH mRNA expression and serum TSH and T4 levels, in addition to affecting thyroid morphology. These findings demonstrate that TBT disrupts the HPT axis. Additionally, the changes found in thyroid hormones suggest that TBT may interfere with the peripheral metabolism of these hormones, an idea corroborated by the observed changes in Dio1 mRNA levels. Therefore, TBT exposition might interfere not only with the thyroid axis but also with thyroid hormone metabolism.

Induction of Type 2 Iodothyronine Deiodinase After Status Epilepticus Modifies Hippocampal Gene Expression in Male Mice

Status epilepticus (SE) is an abnormally prolonged seizure that results from either a failure of mechanisms that terminate seizures or from initiating mechanisms that inherently lead to prolonged seizures. Here we report that mice experiencing a 3 hours of SE caused by pilocarpine exhibit a rapid increase in expression of type 2 iodothyronine deiodinase gene (Dio2) and a decrease in the expression of type 3 iodothyronine deiodinase gene in hippocampus, amygdala and prefrontal cortex. Type 3 iodothyronine deiodinase in hippocampal sections was seen concentrated in the neuronal nuclei, typical of ischemic injury of the brain. An unbiased analysis of the hippocampal transcriptome of mice undergoing 3 hours of SE revealed a number of genes, including those involved with response to oxidative stress, cellular homeostasis, cell signaling, and mitochondrial structure. In contrast, in mice with targeted disruption of Dio2 in astrocytes (Astro D2KO mouse), the highly induced genes in the hippocampus were related to inflammation, apoptosis, and cell death. We propose that Dio2 induction caused by SE accelerates production of T3 in different areas of the central nervous system and modifies the hippocampal gene expression profile, affecting the balance between adaptive and maladaptive mechanisms.

American Thyroid Association Guidelines and Statements: Past, Present, and Future

BACKGROUND

The American Thyroid Association (ATA) is continually striving to improve the quality of its publications. The ATA Guidelines Policies and Procedures Task Force was active during 2017. It recently recommended convening a formal standing committee to review and update policies and procedures for the development of clinical practice guidelines (CPGs) and Statements on an ongoing basis.

OBJECTIVE

This statement reviews the history of official ATA publications and discusses the challenges and findings identified by the Task Force. We also wish to present our "work in progress" and propose future directions for the new ATA Guidelines and Statements Committee (ATA GSC).

METHODS

Our Task Force reviewed the publication record of the ATA with respect to CPGs. We also reviewed existing ATA policies for CPGs and other official statements, examined policies of other organizations, solicited input from external experts and organizations, and convened five conference calls and two in-person meetings.

RESULTS

The ATA has a rich history of developing official publications that have been influential based on download and citation records as well as changes in practice trends. Key future issues to be further addressed by the ATA GSC include the following: (i) striving to improve the methodologic rigor of development of CPGs while balancing considerations of feasibility and timeliness and the role of transparently communicated expert opinion; (ii) formalizing a framework and process for development of new Statements; (iii) increasing stringency and transparency of management of competing interests of individuals being considered for CPG/Statement panel membership; (iv) encouraging consideration of equity and diversity in CPG/Statement development group composition; (v) increasing relevant stakeholder representation (including patient representatives) in development of CPGs/Statements; and (vi) expanding future guideline implementation strategies.

CONCLUSIONS

As shown by the completed literature search, the ATA has a long history of producing CPGs and Statements with global impact on informing clinical management, education, and research in thyroid diseases. The ATA remains committed to a process of continual improvement of its publications and to meeting stakeholder information needs. Based on the work of our Task Force, we have identified many elements that are needed to achieve this goal and areas of challenge for our new committee.

Partial thyrocyte-specific Gαs deficiency leads to rapid-onset hypothyroidism, hyperplasia, and papillary thyroid carcinoma-like lesions in mice

Thyroid function is controlled by thyroid-stimulating hormone (TSH), which binds to its G protein-coupled receptor [thyroid-stimulating hormone receptor (TSHR)] on thyrocytes. TSHR can potentially couple to all G protein families, but it mainly activates the Gs- and Gq/11-mediated signaling cascades. To date, there is a knowledge gap concerning the role of the individual G protein cascades in thyroid pathophysiology. Here, we demonstrate that the thyrocyte-specific deletion of Gs-protein α subunit (Gαs) in adult mice [tamoxifen-inducible Gs protein α subunit deficient (iTGαsKO) mice] rapidly impairs thyrocyte function and leads to hypothyroidism. Consequently, iTGαsKO mice show reduced food intake and activity. However, body weight and the amount of white adipose tissue were decreased only in male iTGαsKO mice. Unexpectedly, hyperplastic follicles and papillary thyroid cancer-like tumor lesions with increased proliferation and slightly increased phospho-ERK1/2 staining were found in iTGαsKO mice at an older age. These tumors developed from nonrecombined thyrocytes still expressing Gαs in the presence of highly elevated serum TSH. In summary, we report that partial thyrocyte-specific Gαs deletion leads to hypothyroidism but also to tumor development in thyrocytes with remaining Gαs expression. Thus, these mice are a novel model to elucidate the pathophysiological consequences of hypothyroidism and TSHR/Gs/cAMP-mediated tumorigenesis.-Patyra, K., Jaeschke, H., Löf, C., Jännäri, M., Ruohonen, S. T., Undeutsch, H., Khalil, M., Kero, A., Poutanen, M., Toppari, J., Chen, M., Weinstein, L. S., Paschke, R., Kero, J. Partial thyrocyte-specific Gαs deficiency leads to rapid-onset hypothyroidism, hyperplasia, and papillary thyroid carcinoma-like lesions in mice.

Red palm oil supplementation does not increase blood glucose or serum lipids levels in Wistar rats with different thyroid status

Red palm oil (RPO) is a rich natural source of antioxidant vitamins, namely carotenes, tocopherols and tocotrienols. However, it contains approximately 50 % saturated fatty acids the regular consumption of which could negatively modify lipid profile. The aim of our study was to test whether 7 weeks of RPO supplementation (1 g/kg body weight/day) would affect blood glucose and lipid metabolism in adult male Wistar rats with altered thyroid status. We induced hypothyroidism and hyperthyroidism in rats by oral administration of either methimazole or mixture of thyroid hormones. Different thyroid status (EU - euthyroid, HY - hypothyroid and HT - hyperthyroid) was characterized by different serum thyroid hormones levels (total and free thyroxine and triiodothyronine), changes in the activity of a marker enzyme of thyroid status - liver mitochondrial glycerol-3-phosphate dehydrogenase, and altered absolute and relative heart weights. Fasting blood glucose levels were higher in HT rats in comparison with EU and HY rats, but the changes caused by RPO supplementation were not significant. The achievement of the HY status significantly increased serum levels of total cholesterol, as well as with high-density lipoprotein-cholesterol and low-density lipoprotein-cholesterol: 2.43+/-0.15, 1.48+/-0.09, 0.89+/-0.08 mmol/l, compared to EU: 1.14+/-0.06, 0.77+/-0.06, 0.34+/-0.05 mmol/l and HT: 1.01+/-0.06, 0.69+/-0.04, 0.20+/-0.03 mmol/l, respectively. RPO supplementation did not increase significantly levels of blood lipids but tended to increase glutathione levels in the liver. In conclusion, RPO supplementation did not induce the presumed deterioration of glucose and lipid metabolism in rats with three well-characterized alterations in thyroid status.

Retinal S-opsin dominance in Ansell's mole-rats (Fukomys anselli) is a consequence of naturally low serum thyroxine

Mammals usually possess a majority of medium-wavelength sensitive (M-) and a minority of short-wavelength sensitive (S-) opsins in the retina, enabling dichromatic vision. Unexpectedly, subterranean rodents from the genus Fukomys exhibit an S-opsin majority, which is exceptional among mammals, albeit with no apparent adaptive value. Because thyroid hormones (THs) are pivotal for M-opsin expression and metabolic rate regulation, we have, for the first time, manipulated TH levels in the Ansell's mole-rat (Fukomys anselli) using osmotic pumps. In Ansell's mole-rats, the TH thyroxine (T4) is naturally low, likely as an adaptation to the harsh subterranean ecological conditions by keeping resting metabolic rate (RMR) low. We measured gene expression levels in the eye, RMR, and body mass (BM) in TH-treated animals. T4 treatment increased both, S- and M-opsin expression, albeit M-opsin expression at a higher degree. However, this plasticity was only given in animals up to approximately 2.5 years. Mass-specific RMR was not affected following T4 treatment, although BM decreased. Furthermore, the T4 inactivation rate is naturally higher in F. anselli compared to laboratory rodents. This is the first experimental evidence that the S-opsin majority in Ansell's mole-rats is a side effect of low T4, which is downregulated to keep RMR low.

A High-Fat Diet Rich in Saturated and Mono-Unsaturated Fatty Acids Induces Disturbance of Thyroid Lipid Profile and Hypothyroxinemia in Male Rats

SCOPE

Increasing evidence has shown that the disturbance of lipid metabolism might make a possible contribution to the pathogenesis of organ dysfunction, including thyroid, yet it is unknown whether excess intake of dietary fat interferes in thyroid lipid profile. We investigate the effects of dietary fat toward the thyroid lipid profile and thyroid function.

METHODS AND RESULTS

Male Sprague-Dawley rats are fed with high-fat diet (HFD) rich in saturated and mono-unsaturated fatty acids or chow diet for 18 weeks. LC-MS analysis of thyroid shows that total free fatty acids (FFAs) content is significantly higher in HFD rats. The concentration of highly saturated triglycerides significantly increases in HFD rats, whereas the polyunsaturated triglyceride significantly decreases, indicating the decrease in unsaturation in the HFD group. Significant increase of lysophosphatidylcholines (LPC) is observed in HFD rats. Thyroid function tests show hypothyroxinemia (total thyroxine [TT₄ ] and free thyroxine [FT₄ ]) in HFD rats, and elevated thyrotropin (TSH) concentration. The HFD rats also show decreased thyroid uptake of iodine.

CONCLUSION

Excess intake of dietary fat induces disturbance of thyroid lipid profile and hypothyroxinemia, indicating thyroid dysfunction. We speculate that it may provide a new prospect in understanding the pathogenesis of hypothyroidism.

A Transgenic Mouse Model for Detection of Tissue-Specific Thyroid Hormone Action

Thyroid hormone (TH) is present in the systemic circulation and thus should affect all cells similarly in the body. However, tissues have a complex machinery that allows tissue-specific optimization of local TH action that calls for the assessment of TH action in a tissue-specific manner. Here, we report the creation of a TH action indicator (THAI) mouse model to study tissue-specific TH action. The model uses a firefly luciferase reporter readout in the context of an intact transcriptional apparatus and all elements of TH metabolism and transport and signaling. The THAI mouse allows the assessment of the changes of TH signaling in tissue samples or in live animals using bioluminescence, both in hypothyroidism and hyperthyroidism. Beyond pharmacologically manipulated TH levels, the THAI mouse is sufficiently sensitive to detect deiodinase-mediated changes of TH action in the interscapular brown adipose tissue (BAT) that preserves thermal homeostasis during cold stress. The model revealed that in contrast to the cold-induced changes of TH action in the BAT, the TH action in this tissue, at room temperature, is independent of noradrenergic signaling. Our data demonstrate that the THAI mouse can also be used to test TH receptor isoform-specific TH action. Thus, THAI mouse constitutes a unique model to study tissue-specific TH action within a physiological/pathophysiological context and test the performance of thyromimetics. In conclusion, THAI mouse provides an in vivo model to assess a high degree of tissue specificity of TH signaling, allowing alteration of tissue function in health and disease, independently of changes in circulating levels of TH.

Chronic infusion of taurolithocholate into the brain increases fat oxidation in mice

Bile acids can function in the postprandial state as circulating signaling molecules in the regulation of glucose and lipid metabolism via the transmembrane receptor TGR5 and nuclear receptor FXR. Both receptors are present in the central nervous system, but their function in the brain is unclear. Therefore, we investigated the effects of intracerebroventricular (i.c.v.) administration of taurolithocholate (tLCA), a strong TGR5 agonist, and GW4064, a synthetic FXR agonist, on energy metabolism. We determined the effects of chronic i.c.v. infusion of tLCA, GW4064, or vehicle on energy expenditure, body weight and composition as well as tissue specific fatty acid uptake in mice equipped with osmotic minipumps. We found that i.c.v. administration of tLCA (final concentration in cerebrospinal fluid: 1 μM) increased fat oxidation (tLCA group: 0.083 ± 0.006 vs control group: 0.036 ± 0.023 kcal/h, F = 5.46, P = 0.04) and decreased fat mass (after 9 days of tLCA infusion: 1.35 ± 0.13 vs controls: 1.96 ± 0.23 g, P = 0.03). These changes were associated with enhanced uptake of triglyceride-derived fatty acids by brown adipose tissue and with browning of subcutaneous white adipose tissue. I.c.v. administration of GW4064 (final concentration in cerebrospinal fluid: 10 μM) did not affect energy metabolism, body composition nor bile acid levels, negating a role of FXR in the central nervous system in metabolic control. In conclusion, bile acids such as tLCA may exert metabolic effects on fat metabolism via the brain.

Effects of growth hormone on thyroid function are mediated by type 2 iodothyronine deiodinase in humans

PURPOSE

Growth hormone (GH) therapy in adults alters thyroid function, and acromegaly often involves thyroid disease. The present study aimed to elucidate roles and mechanisms of GH in regulating thyroid function.

METHODS

We performed two retrospective observational studies, which focused on consecutive patients with severe adult GH deficiency who received recombinant human GH (rhGH) therapy (n = 20) and consecutive patients with acromegaly who underwent transsphenoidal surgery (TSS) (n = 25). In both studies, serum free triiodothyronine (fT3), free thyroxine (fT4), and fT3/fT4 ratio were examined before and after the interventions. We subsequently administered GH to four human cell lines (HepG2, TSA201, MCF7, and HTC/C3) in vitro, and examined changes in mRNA levels of iodothyronine deiodinases (D1, D2, and D3).

RESULTS

Median serum fT3 level significantly increased after rhGH therapy from 2.38 to 2.78 pg/mL (p < 0.001), and fT4 decreased from 1.115 to 1.065 ng/dL (p = 0.081). TSS significantly decreased median serum fT3 from 3.03 to 2.53 pg/mL (p < 0.001), and increased fT4 from 1.230 to 1.370 ng/dL (p < 0.001). In vitro, GH significantly increased D2 expression at the mRNA level in HTC/C3 cells (p < 0.01), as well as D2 protein and its activity.

CONCLUSIONS

GH increased serum fT3 level and decreased serum fT4 level in humans. Our results suggest that its mechanism involves D2 upregulation. Considering this GH effect on thyroid hormone metabolism, data on thyroid function could be useful in the management of GH deficiency and acromegaly.

Cardiac ischemia/reperfusion injury is inversely affected by thyroid hormones excess or deficiency in male Wistar rats

AIM

Thyroid dysfunctions can increase the risk of myocardial ischemia and infarction. However, the repercussions on cardiac ischemia/reperfusion (IR) injury remain unclear so far. We report here the effects of hypothyroidism and thyrotoxicosis in the susceptibility to IR injury in isolated rat hearts compared to euthyroid condition and the potential role of antioxidant enzymes.

METHODS

Hypothyroidism and thyrotoxicosis were induced by administration of methimazole (MMZ, 300 mg/L) and thyroxine (T4, 12 mg/L), respectively in drinking water for 35 days. Isolated hearts were submitted to IR and evaluated for mechanical dysfunctions and infarct size. Superoxide dismutase types 1 and 2 (SOD1 and SOD2), glutathione peroxidase types 1 and 3 (GPX 1 and GPX3) and catalase mRNA levels were assessed by quantitative RT-PCR to investigate the potential role of antioxidant enzymes.

RESULTS

Thyrotoxicosis elicited cardiac hypertrophy and increased baseline mechanical performance, including increased left ventricle (LV) systolic pressure, LV developed pressure and derivatives of pressure (dP/dt), whereas in hypothyroid hearts exhibited decreased dP/dt. Post-ischemic recovery of LV end-diastolic pressure (LVEDP), LVDP and dP/dt was impaired in thyrotoxic rat hearts, whereas hypothyroid hearts exhibited improved LVEDP and decreased infarct size. Catalase expression was decreased by thyrotoxicosis.

CONCLUSION

Thyrotoxicosis was correlated, at least in part, to cardiac remodeling and increased susceptibility to IR injury possibly due to down-regulation of antioxidant enzymes, whereas hypothyroid hearts were less vulnerable to IR injury.

The Deiodinase Trio and Thyroid Hormone Signaling

Thyroid hormone signaling is customized in a time and cell-specific manner by the deiodinases, homodimeric thioredoxin fold containing selenoproteins. This ensures adequate T3 action in developing tissues, healthy adults and many disease states. D2 activates thyroid hormone by converting the pro-hormone T4 to T3, the biologically active thyroid hormone. D2 expression is tightly regulated by transcriptional mechanisms triggered by endogenous as well as environmental cues. There is also an on/off switch mechanism that controls D2 activity that is triggered by catalysis and functions via D2 ubiquitination/deubiquitination. D3 terminates thyroid hormone action by inactivation of both T4 and T3 molecules. Deiodinases play a role in thyroid hormone homeostasis, development, growth and metabolic control by affecting the intracellular levels of T3 and thus gene expression on a cell-specific basis. In many cases, tight control of these pathways by T3 is achieved with coordinated reciprocal changes in D2-mediated thyroid hormone activation D3-mediated thyroid hormone inactivation.

Novel immunoassay for TSH measurement in rats

Measuring thyroid hormones is an important aspect for the study of metabolism and for monitoring diseases in both human and animal models. The traditional method for hormone measurement in rats is the radioimmunoassay (RIA). However, the RIA is associated with some practical disadvantages, including the use of radioactive material, the need for specialized equipment and expert staff, the short shelf-life of kits according to the half-life of the radioisotope and high costs. The objective of this study was to develop a new cost-effective method for measuring TSH levels in rats that avoids the use of radioactive material. We developed an in-house competitive immunoassay using a reference standard, polyclonal antibody produced in rabbits and biotinylated antigen. This method was tested in 64 Wistar rats that were divided into a control group (n = 41) and a group with hypothyroidism (n = 23). Our assay demonstrated an analytical sensitivity of 0.24 ng/mL (n = 12) and an intra-assay coefficient of variation (CV) of 8.9% for sera with TSH levels of 1.5 ng/mL and 13.2% for sera with TSH levels of 17.5 ng/mL (n = 14). The inter-assay CV was 13.5% for sera with TSH levels of 1.4 ng/mL and 14.5% for TSH levels of 18.2 ng/mL (n = 5). The analysis of mean TSH levels in control rats (5.06 ± 0.5701) and hypothyroid rats (51.09 ± 5.136) revealed a statistically significant difference (p < 0.001) between the groups. This method showed good sensitivity, can be automated and is low-cost compared with RIA. Our method offers a viable alternative for TSH measurement in rats.

Adult neural stem cell fate is determined by thyroid hormone activation of mitochondrial metabolism

Objective

In the adult brain, neural stem cells (NSCs) located in the subventricular zone (SVZ) produce both neuronal and glial cells. Thyroid hormones (THs) regulate adult NSC differentiation towards a neuronal phenotype, but also have major roles in mitochondrial metabolism. As NSC metabolism relies mainly on glycolysis, whereas mature cells preferentially use oxidative phosphorylation, we studied how THs and mitochondrial metabolism interact on NSC fate determination.

Methods

We used a mitochondrial membrane potential marker in vivo to analyze mitochondrial activity in the different cell types in the SVZ of euthyroid and hypothyroid mice. Using primary adult NSC cultures, we analyzed ROS production, SIRT1 expression, and phosphorylation of DRP1 (a mitochondrial fission mediator) as a function of TH availability.

Results

We observed significantly higher mitochondrial activity in cells adopting a neuronal phenotype in vivo in euthyroid mice. However, prolonged hypothyroidism reduced not only neuroblast numbers but also their mitochondrial activity. In vitro studies showed that TH availability favored a neuronal phenotype and that blocking mitochondrial respiration abrogated TH-induced neuronal fate determination. DRP1 phosphorylation was preferentially activated in cells within the neuronal lineage and was stimulated by TH availability.

Conclusions

These results indicate that THs favor NSC fate choice towards a neuronal phenotype in the adult mouse SVZ through effects on mitochondrial metabolism.

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Thyroid hormones (TH) favor neuronal fate decision in the adult sub-ventricular zone (SVZ).

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Mitochondrial activity and ROS production are higher in cells differentiating to neuronal fate.

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TH activate the fission-inducing factor DRP1 in cells acquiring a neuronal fate.

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TH favor a neuronal fate in the adult SVZ through induction of mitochondrial respiration.

An extremely high dietary iodide supply forestalls severe hypothyroidism in Na+/I- symporter (NIS) knockout mice

The sodium/iodide symporter (NIS) mediates active iodide (I⁻) accumulation in the thyroid, the first step in thyroid hormone (TH) biosynthesis. Mutations in the SLC5A5 gene encoding NIS that result in a non-functional protein lead to congenital hypothyroidism due to I⁻ transport defect (ITD). ITD is a rare autosomal disorder that, if not treated promptly in infancy, can cause mental retardation, as the TH decrease results in improper development of the nervous system. However, in some patients, hypothyroidism has been ameliorated by unusually large amounts of dietary I⁻. Here we report the first NIS knockout (KO) mouse model, obtained by targeting exons 6 and 7 of the Slc5a5 gene. In NIS KO mice, in the thyroid, stomach, and salivary gland, NIS is absent, and hence there is no active accumulation of the NIS substrate pertechnetate (^99mTcO₄⁻). NIS KO mice showed undetectable serum T₄ and very low serum T₃ levels when fed a diet supplying the minimum I⁻ requirement for rodents. These hypothyroid mice displayed oxidative stress in the thyroid, but not in the brown adipose tissue or liver. Feeding the mice a high-I⁻ diet partially rescued TH biosynthesis, demonstrating that, at high I⁻ concentrations, I⁻ enters the thyroid through routes other than NIS.

Germain DL, Hernandez A, Forrest D. Deletion of the Thyroid Hormone-Activating Type 2 Deiodinase Rescues Cone Photoreceptor Degeneration but Not Deafness in Mice Lacking Type 3 Deiodinase

Type 2 deiodinase amplifies and type 3 deiodinase depletes levels of the active form of thyroid hormone, triiodothyronine. Given the opposing activities of these enzymes, we tested the hypothesis that they counteract each other's developmental functions by investigating whether deletion of type 2 deiodinase (encoded by Dio2) modifies sensory phenotypes in type 3 deiodinase-deficient (Dio3-/-) mice. Dio3-/- mice display degeneration of retinal cones, the photoreceptors that mediate daylight and color vision. In Dio2-/- mice, cone function was largely normal but deletion of Dio2 in Dio3-/- mice markedly recovered cone numbers and electroretinogram responses, suggesting counterbalancing roles for both enzymes in cone survival. Both Dio3-/- and Dio2-/- strains exhibit deafness with cochlear abnormalities. In Dio3-/-;Dio2-/- mice, deafness was exacerbated rather than alleviated, suggesting unevenly balanced actions by these enzymes during auditory development. Dio3-/- mice also exhibit an atrophic thyroid gland, low thyroxine, and high triiodothyronine levels, but this phenotype was ameliorated in Dio3-/-;Dio2-/- mice, indicating counterbalancing roles for the enzymes in determining the thyroid hormone status. The results suggest that the composite action of these two enzymes is a critical determinant in visual and auditory development and in setting the systemic thyroid hormone status.

Changes in Hepatic TRβ Protein Expression, Lipogenic Gene Expression, and Long-Chain Acylcarnitine Levels During Chronic Hyperthyroidism and Triiodothyronine Withdrawal in a Mouse Model

BACKGROUND

Thyroid hormone (TH) has important roles in regulating hepatic metabolism. It was previously reported that most hepatic genes activated by a single triiodothyronine (T3) injection became desensitized after multiple injections, and that approximately 10% of target genes did not return to basal expression levels after T3 withdrawal, despite normalization of serum TH and thyrotropin (TSH) levels. To determine the possible mechanism(s) for desensitization and incomplete recovery of hepatic target gene transcription and their effects on metabolism, mRNA and/or protein expression levels of key regulators of TH action were measured, as well as metabolomic changes after chronic T3 treatment and withdrawal.

METHODS

Adult male mice were treated with daily injections of T3 (20 μg/100 g body weight) for 14 days followed by the cessation of T3 for 10 days. Livers were harvested at 6 hours, 24 hours, and 14 days after the first T3 injection, and at 10 days after withdrawal, and then analyzed by quantitative reverse transcription polymerase chain reaction, Western blotting, and metabolomics.

RESULTS

Although TH receptor (TRα and TRβ) mRNAs decreased slightly after chronic T3 treatment, only TRβ protein decreased before returning to basal expression level after withdrawal. The expression of other regulators of TH action was unchanged. TRβ protein expression was also decreased in adult male monocarboxylate transporter-8 (Mct8)-knockout mice, an in vivo model of chronic intrahepatic hyperthyroidism. Previously, increased hepatic long-chain acylcarnitine levels were found after acute TH treatment. However, in this study, long-chain acylcarnitine levels were unchanged after chronic T3, and paradoxically increased after T3 withdrawal. Pathway analyses of the previous microarray results showed upregulation of lipogenic genes after acute T3 treatment and withdrawal. Phosphorylation of acetyl-CoA carboxylase also decreased after T3 withdrawal.

CONCLUSIONS

Decreased hepatic TRβ protein expression occurred after chronic T3 exposure in adult male wild-type and Mct8-knockout mice. Gene array pathway and metabolomics analyses showed abnormalities in hepatic lipogenic gene expression and acylcarnitine levels, respectively, after withdrawal, despite normalization of serum TSH and TH levels. These findings may help explain the variable clinical presentations of some patients during hyperthyroidism and recovery, since TRβ protein, target gene expression, and metabolic adaptive changes can occur in individual tissues without necessarily being reflected by circulating TH and TSH concentrations.

A Thyroid Hormone-Based Strategy for Correcting the Biochemical Abnormality in X-Linked Adrenoleukodystrophy

X-linked adrenoleukodystrophy (X-ALD) is a rare, genetic disorder characterized by adrenal insufficiency and central nervous system (CNS) demyelination. All patients with X-ALD have the biochemical abnormality of elevated blood and tissue levels of very long chain fatty acids (VLCFAs), saturated fatty acids with 24 to 26 carbons. X-ALD results from loss of function mutations in the gene encoding the peroxisomal transporter ABCD1, which is responsible for uptake of VLCFAs into peroxisomes for degradation by oxidation. One proposed therapeutic strategy for genetic complementation of ABCD1 is pharmacologic upregulation of ABCD2, a gene encoding a homologous peroxisomal transporter. Here, we show that thyroid hormone or sobetirome, a clinical-stage selective thyroid hormone receptor agonist, increases cerebral Abcd2 and lowers VLCFAs in blood, peripheral organs, and brains of mice with defective Abcd1. These results support an approach to treating X-ALD that involves a thyromimetic agent that reactivates VLCFA disposal both in the periphery and the CNS.