Hypothyroidism Induces Hypophagia Associated with Alterations in Protein Expression of Neuropeptide Y and Proopiomelanocortin in the Arcuate Nucleus, Independently of Hypothalamic Nuclei-Specific Changes in Leptin Signaling

Maternal obesity induces sex-specific changes in the endocannabinoid system of the hypothalamus and dorsal hippocampus of offspring associated with anxiety-like behavior in adolescent female rats

Maternal obesity during perinatal period increases the risk of metabolic and behavioral deleterious outcomes in the offspring, since it is critical for brain development, maturation, and reorganization. These processes are highly modulated by the endocannabinoid system (ECS), which comprises the main lipid ligands anandamide and 2-arachidonoylglycerol, cannabinoid receptors 1 and 2 (CB1R and CB2R), and several metabolizing enzymes. The ECS is overactivated in obesity and it contributes to the physiological activity of the hypothalamus-pituitary-adrenal (HPA) axis, promoting stress relief. We have previously demonstrated that maternal high-fat diet during gestation and lactation programmed the food preference for fat in adolescent male offspring and adult male and female offspring. In the present study, we hypothesized that maternal diet-induced obesity would induce sex-specific changes of the ECS in the hypothalamus and dorsal hippocampus of rat offspring associated with dysregulation of the HPA axis and stress-related behavior in adolescence. Rat dams were fed a control (C) or an obesogenic high-fat high-sugar diet (OD) for nine weeks prior to mating and throughout gestation and lactation. Maternal obesity differentially altered the CB1R in the hypothalamus of neonate offspring, with significant increase in male but not in female pups, associated with decreased CB2R prior to obesity development. In adolescence, maternal obesity induced anxiety-like behavior only in adolescent females which was associated with increased content of CB1R in the dorsal hippocampus. Our findings suggest that the early origins of anxiety disorders induced by maternal exposome is associated with dysregulation of the brain ECS, with females being more susceptible.

Testis-Specific Protein, Y-Encoded-Like 2 Activates JAK2/STAT3 Pathway in Hypothalamic Paraventricular Nucleus to Sustain Hypertension

BACKGROUND

In the hypothalamic paraventricular nucleus (PVN) of spontaneously hypertensive rats (SHRs), the expression of the testis-specific protein, Y-encoded-like 2 (TSPYL2) and the phosphorylation level of Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) are higher comparing with the normotensive Wistar Kyoto rats (WKY). But how they are involved in hypertension remains unclear. TSPYL2 may interact with JAK2/STAT3 in PVN to sustain high blood pressure during hypertension.

METHODS

Knockdown of TSPYL2 via adeno-associated virus (AAV) carrying shRNA was conducted through bilateral microinjection into the PVN of SHR and WKY rats. JAK2/STAT3 inhibition was achieved by intraperitoneally or PVN injection of AG490 into the SHRs. Blood pressure (BP), plasma norepinephrine (NE), PVN inflammatory response, and PVN oxidative stress were measured.

RESULTS

TSPYL2 knock-down in the PVN of SHRs but not WKYs led to reduced BP and plasma NE, deactivation of JAK2/STAT3, decreased expression of pro-inflammatory cytokine IL-1β, and increased expression of anti-inflammatory cytokine IL-10 in the PVN. Meanwhile, AG490 administrated in both ways reduced the BP in the SHRs and deactivated JAK2/STAT3 but failed to change the expression of TSPYL2 in PVN. AG490 also downregulated expression of IL-1β and upregulated expression of IL-10. Both knockdown of TSPYL2 and inhibition of JAK2/STAT3 can reduce the oxidative stress in the PVN of SHRs.

CONCLUSION

JAK2/STAT3 is regulated by TSPYL2 in the PVN of SHRs, and PVN TSPYL2/JAK2/STAT3 is essential for maintaining high BP in hypertensive rats, making it a potential therapeutic target for hypertension.

Maternal high-fat diet decreases milk endocannabinoids with sex-specific changes in the cannabinoid and dopamine signaling and food preference in rat offspring

INTRODUCTION

Maternal high-fat (HF) diet during gestation and lactation programs obesity in rat offspring associated with sex-dependent and tissue-specific changes of the endocannabinoid system (ECS). The ECS activation induces food intake and preference for fat as well as lipogenesis. We hypothesized that maternal HF diet would increase the lipid endocannabinoid levels in breast milk programming cannabinoid and dopamine signaling and food preference in rat offspring.

METHODS

Female Wistar rats were assigned into two experimental groups: control group (C), which received a standard diet (10% fat), or HF group, which received a high-fat diet (29% fat) for 8 weeks before mating and during gestation and lactation. Milk samples were collected to measure endocannabinoids and fatty acids by mass spectrometry. Cannabinoid and dopamine signaling were evaluated in the nucleus accumbens (NAc) of male and female weanling offspring. C and HF offspring received C diet after weaning and food preference was assessed in adolescence.

RESULTS

Maternal HF diet reduced the milk content of anandamide (AEA) (p<0.05) and 2-arachidonoylglycerol (2-AG) (p<0.05). In parallel, maternal HF diet increased adiposity in male (p<0.05) and female offspring (p<0.05) at weaning. Maternal HF diet increased cannabinoid and dopamine signaling in the NAc only in male offspring (p<0.05), which was associated with higher preference for fat in adolescence (p<0.05).

CONCLUSION

Contrary to our hypothesis, maternal HF diet reduced AEA and 2-AG in breast milk. We speculate that decreased endocannabinoid exposure during lactation may induce sex-dependent adaptive changes of the cannabinoid-dopamine crosstalk signaling in the developing NAc, contributing to alterations in neurodevelopment and programming of preference for fat in adolescent male offspring.

Effects of Extreme Light Cycle and Density on Melatonin, Appetite, and Energy Metabolism of the Soft-Shelled Turtle (Pelodiscus sinensis)

Constant darkness and constant light exposure often disturb the circadian rhythm in the behavior and energy metabolism of vertebrates. Melatonin is known as the hormonal mediator of photoperiodic information to the central nervous system and plays a key role in food intake and energy balance regulation in vertebrates. The popularly cultured soft-shelled turtle Pelodiscus sinensis has been reported to grow better under constant darkness; however, the underlying physiological mechanism by which darkness benefits turtle growth is not clear yet. We hypothesized that increased melatonin levels induced by darkness would increase appetite and energy metabolism and thus promote growth in P. sinensis. In addition, in order to elucidate the interaction of photoperiod and density, juvenile turtles were treated under three photoperiods (light/dark cycle: 24L:0D, 12L:12D, 0L:24D, light density 900 lux) and two stocking densities (high density: 38.10 ind./m2, low density: 6.35 ind./m2) for 4 weeks, and then the blood and brain tissues of turtles were collected during the day (11:00–13:00) and at night (23:00–1:00) after 2 days of fasting. We examined changes in plasma melatonin levels, food intake (FI), and appetite-related hormones (plasma ghrelin and leptin), as well as growth and energy metabolism parameters such as specific growth rate (SGR), standard metabolic rate (SMR), plasma growth hormone (GH), and thyroid hormone/enzyme activity (plasma triiodothyronine T3, thyroxine T4, and T45′-deiodinase activity). Moreover, we also assessed the responses of mRNA expression levels of food intake-related genes (kisspeptin 1 (Kiss1); cocaine amphetamine-regulated transcript (CART); neuropeptide Y (NPY)) in the brain. The results showed that under high density, SGR was the lowest in 24L:0D and the highest in 0L:24D. FI was the highest in 0L:24D regardless of density. Plasma melatonin was the highest in 0L:24D under high density at night. SMR increased with decreasing light time regardless of density. Most expressions of the measured appetite-related genes (Kiss1, CART, and NPY) were not affected by photoperiod, nor were the related hormone levels, such as plasma leptin, ghrelin, and GH. However, thyroid hormones were clearly affected by photoperiod. T3 level in 0L:24D under high density during the day was the highest among all treatment groups. T4 in 24L:0D under high density during the day and T45′-deiodinase activity in 24L:0D under low density at night were significantly reduced compared with the control. Furthermore, the energy metabolism-related hormone levels were higher under higher density, especially during the day. Together, melatonin secretion is not only modulated by light but also likely to be regulated by unknown endogenous factors and density. Altered plasma melatonin induced by constant darkness and density seems to be involved in the modulation of energy metabolism rather than appetite in the soft-shelled turtle.

Analysis of the transcriptional activity of genes of neuropeptides and their receptors in the blood of patients with thyroid pathology

The thyroid hormone plays a vital role in the development and maturation of the nervous system not only during prenatal and perinatal age but also in adults. “Peripheral marker hypothesis” revealed that gene expression changes in some regions of the brain are reflected into the peripheral blood lymphocytes. The objective of the study was to investigate changes in the gene expression profile of neuropeptides and their receptors in patients with different forms of thyroid pathology. One hundred fifty-three patients with thyroid pathology were enrolled in the study. They were divided into three groups: group 1 included 16 patients with postoperative hypothyroidism, group 2 included 65 patients with hypothyroidism resulting from autoimmune thyroiditis (AIT), and group 3 included 72 patients with AIT and elevated levels of anti-thyroglobulin (anti-Tg) and anti-thyroid peroxidase (anti-TPO) antibodies in the serum. We used a pathway-specific polymerase chain reaction (PCR) array (RT² Profiler™ PCR Array Human Neurotrophins & Receptors, QIAGEN, Germany) to identify and verify neuropeptides and receptors pathway-focused gene expression in 12 individuals that were randomly selected from each group using real-time PCR. Our research identified that patients with postoperative hypothyroidism had a considerably increased expression of NPY1R, NTSR1, and NPY4R. The patients with hypothyroidism caused by autoimmune thyroiditis had considerably lower expression of NTSR1, while the expression of NPY1R increased. The mRNA levels of NPY2R and PNOC increased in the patients with elevated levels of autoantibodies anti-Tg and anti-TPO in the serum, and mRNA levels of NPY1R and NTSR1 decreased in this group of patients.

Thyroxine Treatment During the Perinatal Stage Prevents the Alterations in the ObRb-STAT3 Leptin Signaling Pathway Caused by Congenital Hypothyroidism

Thyroid hormone deficiency during crucial stages of development causes congenital hypothyroidism. This syndrome alters hypothalamic pathways involved in long-term bodyweight regulation as ObRb-STAT3 leptin signaling pathway, which is associated with metabolic syndrome. This study aimed to determine if thyroxine treatment during pregnancy and lactation in hypothyroid mothers avoids, in the congenital hypothyroid offspring, the alterations in metabolic programming related to metabolic syndrome and the ObRb-STAT3 leptin signaling pathway in hypothalamus. Twenty-four virgin female Wistar rats were divided into euthyroid, hypothyroid, and hypothyroid with thyroxine treatment (20 μg/kg/day T₄ since pregnancy until lactation). The bodyweight and energy intake, insulin resistance, glucose tolerance, metabolic and hormonal parameters were determined in offspring at 28 weeks after birth. Then, the rats were euthanized to obtain adipose tissue reserves and hypothalamus to measure the expression of ObRb, STAT3, pSTAT3, and SOCS3. Congenital hypothyroidism presented metabolic syndrome such as insulin resistance, glucose tolerance, dyslipidemias, an increase in cardiovascular risk (Castelli I males:166.67%, females: 173.56%; Castelli II males: 375.51%, females: 546.67%), and hypothalamic leptin resistance (SOCS3, Males: 10.96%, females: 25.85%). Meanwhile, the thyroxine treatment in the mothers during pregnancy and lactation prevents the metabolic disturbance. In conclusion, thyroxine treatment during the critical perinatal stage for metabolic programming prevents congenital hypothyroidism-caused metabolic syndrome and hypothalamic leptin resistance.

Hypothyroidism in restless legs syndrome

The diurnal nature of restless legs syndrome (RLS) and its response to dopamine hint that hormones are central in RLS pathophysiology. Hypothyroidism has been linked to RLS, but studies are limited. This study's objective is to determine whether RLS is more prevalent in persons with hypothyroidism and whether hypothyroidism is more prevalent in RLS sufferers. Persons with hypothyroidism and controls were recruited through an on-line registry of potential research participants. RLS was assessed using the Cambridge-Hopkins questionnaire. RLS persons and controls were recruited through RLS Foundation and on-line registry advertisements and assessed for hypothyroidism by self-report. The International RLS Study Group Severity Scale assessed RLS severity; 266 hypothyroid subjects and 321 controls were comparable in age (52.3 ± 13.4 versus 53.9 ± 11.7 years; p = .14) and gender (91.7% versus 91.3% women; p = .85), as were 354 RLS and 313 controls (59.1 ± 13.2 versus 58.2 ± 13.6 years; p = .41; 80.8% versus 78.3% women; p = .42). Hypothyroid participants versus controls had a significantly higher prevalence of RLS (14.3% versus 8.1%; p = .02). RLS participants versus controls had a significantly higher prevalence of hypothyroidism (22.3% versus. 13.8%; p = .005). RLS severity was similar in persons with and without hypothyroidism. Among 73 persons with RLS and hypothyroidism, 14 previously were hyperthyroid versus 0 of 37 persons with hypothyroidism alone (p = .004). RLS prevalence is increased in individuals with hypothyroidism; hypothyroidism prevalence is increased in individuals with RLS. Persons with hypothyroidism and RLS are significantly more likely than those with hypothyroidism alone to have had hyperthyroidism prior to hypothyroidism. Associations between RLS and thyroid disease may shed light on complex biological mechanisms underlying RLS.

The role of thyroid hormone in metabolism and metabolic syndrome

Metabolic syndrome (MetS) and thyroid dysfunction are common in clinical practice. The objectives of this review are to discuss some proposed mechanisms by which thyroid dysfunctions may lead to MetS, to describe the bidirectional relationship between thyroid hormones (THs) and adiposity and finally, to resume a list of recent studies in humans that evaluated possible associations between thyroid hormone status and MetS or its clinical components. Not solely THs, but also its metabolites regulate metabolic rate, influencing adiposity. The mechanisms enrolled are related to its direct effect on adenosine triphosphate (ATP) utilization, uncoupling synthesis of ATP, mitochondrial biogenesis, and its inotropic and chronotropic effects. THs also act controlling core body temperature, appetite, and sympathetic activity. In a bidirectional way, thyroid function is affected by adiposity. Leptin is one of the hallmarks, but the pro-inflammatory cytokines and also insulin resistance impact thyroid function and perhaps its structure. MetS development and weight gain have been positively associated with thyroid-stimulating hormone (TSH) in several studies. Adverse glucose metabolism may be related to hyperthyroidism, but also to reduction of thyroid function or higher serum TSH, as do abnormal serum triglyceride levels. Hypo- and hyperthyroidism have been related to higher blood pressure (BP), that may be consequence of genomic or nongenomic action of THs on the vasculature and in the heart. In summary, the interaction between THs and components of MetS is complex and not fully understood. More longitudinal studies controlling each of all confounding variables that interact with endpoints or exposure factors are still necessary.

Hypothyroidism Alters the Uterine Lipid Levels in Pregnant Rabbits and Affects the Fetal Size

BACKGROUND

Hypothyroidism has been related to low-weight births, abortion and prematurity, which have been associated with changes in the content of glycogen and vascularization of the placenta. Since hypothyroidism can cause dyslipidemia, it may affect the lipid content in the uterus affecting the development of fetuses.

OBJECTIVE

To investigate the effect of hypothyroidism on the lipid levels in serum and uterus during pregnancy and their possible association with the size of fetuses.

METHOD

Adult female rabbits were grouped in control (n = 6) and hypothyroid (n = 6; treated with methimazole for 29 days before and 19 days after copulation). Food intake and body weight were daily registered. At gestational day 19 (GD19), dams were sacrificed under an overdose of anesthesia. Morphometric measures of fetuses were taken. Total cholesterol (TC), triglyceride (TAG), and glucose concentrations were quantified in blood, uterus and ovaries of dams. The expression of uterine 3β- hydroxysteroid dehydrogenase (3β-HSD) was quantified by Western blot.

RESULTS

Hypothyroidism reduced food intake and body weight of dams, as well as promoted low abdominal diameters of fetuses. It did not induce dyslipidemia and hyperglycemia at GD19 and did not modify the content of lipids in the ovary. However, it reduced the content of TAG and TC in the uterus, which was associated with uterine hyperplasia and an increased expression of 3β-HSD in the uterus.

CONCLUSION

Hypothyroidism alters the lipid content in the uterus that might subsequently affect the energy production and lipid signaling important to fetal development.

The influence of vestibular stimulation on metabolism and body composition

Obesity, diabetes and metabolic disease represent an ongoing and rapidly worsening public health issue in both the developed, and much of the developing world. Although there are many factors that influence fat storage, it has been clearly demonstrated that the homeostatic cornerstone of metabolism lies within the hypothalamus. Moreover, neuronal damage to vital areas of the hypothalamus can drive reregulation or dysregulation of endocrine function, energy expenditure and appetite, thereby promoting a shift in overall metabolic function towards a state of obesity. Therefore, identification of treatments that influence the hypothalamus to improve obesity and associated metabolic diseases has long been a medical goal. Interestingly, evidence from animal studies suggests that activating the vestibular system, specifically the macular gravity receptor, influences the hypothalamus in a way that decreases body fat storage and causes a metabolic shift towards a leaner state. Given that the macular element of the vestibular system has been shown to activate with transdermal electrical stimulation applied to the mastoids, this may be a potential therapeutic approach for obesity, diabetes or related metabolic diseases, whereby repetitive stimulation of the vestibular system influences hypothalamic control of metabolic homeostasis, thereby encouraging decreased fat storage. Here, we present an up-to-date review of the current literature surrounding the vestibular influence of the hypothalamus and associated homeostatic sites in the context of current and novel therapeutic approaches for improved clinical management of obesity and diabetes.

Pathogenesis of hypothyroidism-induced NAFLD: Evidence for a distinct disease entity?

Nonalcoholic fatty liver disease (NAFLD), the most common liver disease worldwide, may be associated with primary hypothyroidism. However, the pathogenesis underlying such an association is complex and not completely understood. Here, we specifically discuss the pathogenic mechanisms potentially involved in hypothyroidism-induced NAFLD. To this end, we summarize the general pathophysiology of thyroid hormones (TH). Next, we analyze the published data from rodent studies by discussing whether hypothyroid rats may develop NAFLD via hyperphagia; whether mitochondria become energetically more efficient; what the overall energy balance is and if diversion of fatty substrates occurs; and the latest advancements in molecular pathogenesis brought about by metabolomics, cell imaging, lipophagy, autophagy and genetically engineered mouse models. Moreover, we discuss the data published regarding humans on the pathogenic role of TH, metabolic syndrome and other risk factors in hypothyroidism-related NAFLD as well as the putative mechanisms underlying the development of NAFLD-related hepatocellular carcinoma in hypothyroidism. In conclusion, although many research questions still remain unanswered, the pathophysiology of hypothyroidism-induced NAFLD makes this a potentially curable and distinct disease entity. However, further studies are needed to better elucidate the underlying mechanisms, and to ascertain whether treatment with either TH or thyromimetic agents improves NAFLD.

Leptin, neuropeptide Y (NPY), melatonin and zinc levels in experimental hypothyroidism and hyperthyroidism: relation with melatonin and the pineal gland

Background Melatonin, an important neurohormone released from the pineal gland, is generally accepted to exercise an inhibitor effect on the thyroid gland. Zinc mediates the effects of many hormones and is found in the structure of numerous hormone receptors. Aim The present study aims to examine the effect of melatonin supplementation and pinealectomy on leptin, neuropeptide Y (NPY), melatonin and zinc levels in rats with hypothyroidism and hyperthyroidism. Methods This study was performed on the 70 male rats. Experimental animals in the study were grouped as follows: control (C); hypothyroidism (PTU); hypothyroidism + melatonin (PTU + M); hypothyroidism + pinealectomy (PTU + Pnx); hyperthyroidism (H); hyperthyroidism + melatonin (H + M) and hyperthyroidism + pinealectomy (H + Pnx). Blood samples collected at the end of 4-week procedures were analyzed to determine melatonin, leptin, NPY and zinc levels. Results It was found that thyroid parameters thyroid stimulating hormone (TSH), free triiodthyronine (FT3), free thyroxine (FT4), total T3 (TT3) and total T4 (TT4) decreased in hypothyroidism groups and increased in the groups with hyperthyroidism. The changes in these hormones remained unaffected by melatonin supplementation and pinealectomy. Melatonin levels rose in hyperthyroidism and fell in hypothyroidism. Leptin and NPY levels increased in both hypothyroidism and hyperthyroidism. Zinc levels, on the other hand, decreased in hypothyroidism and pinealectomy, but increased in hyperthyroidism. Conclusion The results of the study demonstrate that hypothyroidism and hyperthyroidism affect leptin, NPY, melatonin and zinc values in different ways in rats. However, melatonin supplementation and pinealectomy do not have any significant influence on the changes occurring in leptin, NPY and zinc levels in thyroid dysfunction.

Thyroidectomy stimulates glucagon-like peptide-1 secretion and attenuates hepatic steatosis in high-fat fed rats

Thyroid hormones (THs) as a therapeutic intervention to treat obesity has been tried but the effect of THs on body weight and the mechanistic details of which are far from clear. This study was designed to determine and elucidate the mechanistic details of metabolic action of THs in high-fat diet (HFD) fed Sprague Dawley (SD) rats. Rats were made surgically hypothyroid (thyroidectomy, Thx). Body weights and food and water intake profoundly decreased in HFD fed thyroidectomized group (HN Thx). Results showed that delayed insulin response, increased total cholesterol, high-density lipoprotein, and low-density lipoprotein in HN Thx. Unexpectedly, however, Thx reduced serum and hepatic triglyceride concentrations. Further studies revealed that Thx dramatically increased circulating GLP-1 as well as increased expressions of GLP-1 in small intestine. Diminished hepatic expressions of lipogenic genes, were observed in HN Thx group. Beta-catenin and glutamine synthetase, a known target of β-catenin, were up-regulated in the liver of HN Thx group. The expressions of gluconeogenic genes G6P and PCK were reduced in the liver of HN Thx group. The results may suggest that surgery-induced hypothyroidism increases GLP-1, the actions of which may in part be responsible for the reduction in water intake, appetite and hepatic steatosis.

Leptin, NPY, Melatonin and Zinc Levels in Experimental Hypothyroidism and Hyperthyroidism: The Relation to Zinc

Since zinc mediates the effects of many hormones or is found in the structure of numerous hormone receptors, zinc deficiency leads to various functional impairments in the hormone balance. And also thyroid hormones have important activity on metabolism and feeding. NPY and leptin are affective on food intake and regulation of appetite. The present study is conducted to determine how zinc supplementation and deficiency affect thyroid hormones (free and total T3 and T4), melatonin, leptin, and NPY levels in thyroid dysfunction in rats. The experiment groups in the study were formed as follows: Control (C); Hypothyroidism (PTU); Hypothyroidism+Zinc (PTU+Zn); Hypothyroidism+Zinc deficient; Hyperthyroidism (H); Hyperthyroidism+Zinc (H+Zn); and Hyperthyroidism+Zinc deficient. Thyroid hormone parameters (FT₃, FT₄, TT₃, and TT₄) were found to be reduced in hypothyroidism groups and elevated in the hyperthyroidism groups. Melatonin values increased in hyperthyroidism and decreased in hypothyroidism. Leptin and NPY levels both increased in hypo- and hyperthyroidism. Zinc levels, on the other hand, decreased in hypothyroidism and increased in hyperthyroidism. Zinc supplementation, particularly when thyroid function is impaired, has been demonstrated to markedly prevent these changes.

Advances in TRH signaling

The activity of the hypothalamus-pituitary-thyroid axis (HPT) is coordinated by hypophysiotropic thyrotropin releasing hormone (TRH) neurons present in the paraventricular nucleus of the hypothalamus. Hypophysiotropic TRH neurons act as energy sensors. TRH controls the synthesis and release of thyrotropin, which activates the synthesis and secretion of thyroid hormones; in target tissues, transporters and deiodinases control their local availability. Thyroid hormones regulate many functions, including energy homeostasis. This review discusses recent evidence that covers several aspects of TRH role in HPT axis regulation. Knowledge about the mechanisms of TRH signaling has steadily increased. New transcription factors engaged in TRH gene expression have been identified, and advances made on how they interact with signaling pathways and define the dynamics of TRH neurons response to acute and/or long-term influences. Albeit yet incomplete, the relationship of TRH neurons activity with positive energy balance has emerged. The importance of tanycytes as a central relay for the feedback control of the axis, as well as for HPT responses to alterations in energy balance, and other stimuli has been reinforced. Finally, some studies have started to shed light on the interference of prenatal and postnatal stress and nutrition on HPT axis programing, which have confirmed the axis susceptibility to early insults.

NEW INSIGHTS INTO THE HYPOTHALAMIC-PITUITARY-THYROID AXIS

The discovery of thyrotropin releasing hormone (TRH) in 1969 was the definitive step in decoding the hypothalamic-pituitary thyroid (HPT) axis, thereby opening up the era of neuroendocrinology, while it also revolutionized the diagnostic and therapeutic approach to patients with thyroid diseases. TRH, produced in the hypothalamus, is the central regulator of the HPT. It functions via neurons originating in the paraventricular nucleus (PVN), which integrates multiple neuronal and humoral signals and resets the HPT axis according to variations of external and internal environmental conditions. The TRH activates TSH in the pituitary that stimulates the secretion of thyroxine from thyroid which, in turn, exerts a negative feedback on TSH and TRH secretion. However, various factors are involved in the regulation of the HPT axis. Leptin has both indirect and direct effects on TRH regulation, the former by regulating agouti-related peptide (AGRP) in the arcuate nucleus (ARN) that antagonizes the α-MSH stimulatory activity on pro-TRH gene expression in the PVN, and the latter by stimulating hypothalamic TRH expression, TRH transcription via stimulation of pro-convertase 1 and 2 expression, which lead to enhanced processing of pro-TRH into TRH. The interplay of TRH with leptin and the recently reported influence of ghrelin on the HPT axis can alter the setpoint of the axis. The polyphenol resveratrol, as recently observed, exerts an anxiolytic and antidepressant activity in subclinical hypothyroid (SCH) rats. Resveratrol, by decreasing both TSH and TRH mRNA expression, regulates the HPT axis, while in parallel it regulates the Wnt/β-catenin pathway in the hippocampus. These findings open up possibilities for the therapeutic use of resveratrol as coadjuvant, especially in overt and SCH states marked by anxiety and depression. The clinician should be aware of clinical changes that can invalidate the normal regulation of the HPT axis, the most commonly observed being medications and comorbidities.