Weighing in on Thyroid Signaling in the Hypothalamus: Mechanisms and Interface with Metabolic Regulators

Alterations in body temperature, basal metabolic rate, and body weight are cardinal features used to clinically assess individuals with abnormally low or elevated thyroid hormone (TH) levels and are the result of both central and peripheral actions of TH. ¹ The role of the hypothalamus in mediating many of these actions has been well established, although the mechanisms are not fully understood. ^2,3 The regulation of TH synthesis and release from the thyroid gland, by the hypothalamic–pituitary–thyroid (HPT) axis, involve thyrotropin releasing hormone (TRH) in the paraventricular nucleus (PVN) of the hypothalamus. ²

TRH gene expression in the hypothalamus is directly inhibited by triiodothyronine (T3), mediated primarily by TH receptor (THR) beta 2. ⁴ The levels of T3 in the pituitary and hypothalamus are primarily regulated by the type 2 5′-deiodinase, Dio2, which becomes more active as thyroxine (T4) levels fall, and the 5-deiodinase type 3, Dio 3, which converts T4 to the inactive reverse T3. ⁴ Tanycytes, which express Dio2 and Dio3, found in the median eminence at the base of the third ventricle, are specialized glial cells important for hormone sensing and regulation of hypothalamic hormone secretion, including a role in thyroid function changes associated with nonthyroidal illness (NTI). ⁴

The influence of TH on thermoregulation, metabolism, appetite, and food intake, has been linked to actions in the hypothalamus, which also integrate signals of appetite and satiety in the arcuate nucleus, such as neuropeptide Y and agouti-related peptide, as well as peripheral signals of metabolism and fat stores, such as leptin. ³

A trio of studies published in this issue of Thyroid, ^{5 –7} have identified important aspects of TH signaling in the hypothalamus, investigating the impact on a range of metabolic parameters. TH signaling requires the presence of T3, transport of T3 into the cell by specific transporters in some tissues, and the nuclear receptors, alpha and beta (THRA and THRB), as well nuclear receptor cofactors and other transcription regulators. ⁴ These studies address various components of this TH signaling pathway in the hypothalamus, as well as the actions of thyrotropin (TSH), and an analysis of hypothalamic region-specific regulation.

Sentis et al. ⁵ utilized genetic models of THR isoform deletions and mutations to determine which THR isoform mediates TH actions on body temperature regulation and energy metabolism in the hypothalamus. THRB, important for mediating T3 feedback regulation in the PVN, ³ did not play a role in body temperature regulation. Mice expressing a mutant THRA (TRα1+m) had lower body temperature than control, both at room temperature and at the higher rodent thermoneutral temperature of 30°C, which eliminates tail heat loss.

Oral T3 supplementation normalized the body temperature profile of TRα1+m mice, likely by activation of the mutant THRA at a higher level of T3. Direct expression of the dominant-negative THRA selectively in the hypothalamus, through adeno-associated virus transfection, resulted in a reduced body temperature at room temperature and at 30°C. Mice exposed to cold up to 6 hours, a period dominated by muscle shivering for thermogenesis, maintained normal body temperature. Moreover, the authors did not observe a compensatory response in brown adipose tissue (BAT) and tail temperature as possible adaptation, suggesting a central role of THRA in maintaining body temperature.

There were no changes in the expression of thermogenic markers in BAT, and no “browning” of the inguinal or epididymal white adipose tissue, but some upregulation of genes in the soleus muscle. The authors concluded that central THRA signaling in the hypothalamus is important for maintaining body temperature.

Chandrasekar et al. ⁶ studied the impact of TSH signaling on TH transporters and deiodinase enzymes in hypothalamic tanycytes. In addition to mediating hormone responsiveness for hypothalamic hormones, tanycytes express TH transporters (Slc16a2 and Slco1c1), deiodinases (Dio2 and Dio3), and THRs (THRA and THRB) and TSH receptor (TSH-R). It is interesting to note that THRA was the dominant receptor and presents in all areas of the tanycytic layer. The different expression pattern of Dio2, Dio3, Tshr, and Slco1c1 along the tanycytes indicates a region-specific function of tanycyte subpopulations.

This study analyzed the role of TSH signaling in the expression of TH transporters and deiodinases mRNA, and then used siRNA to selectively knock down genes and determine the impact on modulating signaling. TSH stimulation increased the mRNA levels of Slco1c1, Dio2, and Dio3, with little effect on the expression of Slc16a2. This stimulation required expression of TSH-R and CREB (cAMP response element binding protein), indicating PKA-dependent regulation through TSH-R. In addition, the authors showed a PKC-dependent regulation of Slco1c1 and Dio3, and a ERK-mediated increased of Dio3 and decrease of Dio2 expression, through TSH-R.

Subpopulations of tanycytes varied in response to TSH signaling indicating distinct intracellular signaling pathways. The authors conclude that TSH induces transcriptional regulation of genes that modulate TH signaling in tanycytes through the Tshr/Gαq/PKC pathway, in parallel to the Tshr/Gαs/PKA/CREB pathway. These differential actions of TSH on tanycytic subpopulations may influence the regulation of the HPT axis. Further investigations are warranted to explore the physiological consequences of TSH-mediated effects on tanycytes in vivo.

Finally, Ruska et al. ⁷ studied the impact of GLP-1 agonists on TRH-synthesizing neurons in the hypothalamic PVN, investigating the role of GLP-1 signaling on regulating energy expenditure and the potential connection with the HPT axis. They examined the anatomical and functional relationship of TRH neurons and the GLP-1 system, including in vitro studies of neuron mapping and function, as well as metabolic phenotyping in mice. The PVN TRH neurons were shown to be innervated by GLP-1 producing neurons and also shown to express GLP-1 receptor (GLP-1R).

The GLP-1 innervated TRH neurons express GLP-1R, but GLP-1R was also present in the axons of the hypophysiotropic TRH neurons in the blood–brain barrier free median eminence, providing a site of action for peripheral GLP-1. Although GLP-1 increased the firing rate of TRH neurons and directly stimulated GABAergic input in vitro, it also indirectly inhibited the TRH neurons by regulating the activity of inhibitory inputs. In addition, GLP-1 inhibited the release of TRH from the hypophysiotropic axons in the median eminence.

Peripheral administration of GLP-1R agonist in animal studies markedly inhibited the food intake and energy expenditure, but did not influence TRH expression in the PVN and was associated with lower circulating free T4 levels. The authors concluded that GLP-1R activation has a direct stimulatory effect on TRH neurons in the PVN, but GLP-1R may also have inhibitory effects on TRH neurons, as well as the actions of GLP-1 from the peripheral circulation. The in vivo studies, however, did not support a role of GLP-1 agonists on the HPT axis causing the associated weight loss.

These three studies demonstrate the complexity of determining the mechanism of TH signaling, even in a single region of the brain. The role of the THRA isoform in mediating body temperature regulation in the hypothalamus by Sentis et al. is a robust mechanism, and the ability of excess T3 administration to overcome the disruption of a mutant THRA indicates that excess ligand availability can overcome the effects of a mutant receptor. This has been demonstrated in clinical studies of T4 and T3 administration to individuals with resistance to thyroid hormone (RTH)-alpha, who have metabolic manifestations, partially reversed with excess levothyroxine treatment, but this varies by individual and the specific THRA mutation. ⁸

The hypothalamus is another tissue, as has been described for BAT, where THRA and THRB mediate distinct functions within the same tissue. A chemogenetic mapping approach of TRH neurons in the hypothalamus confirmed an essential role for THRB in mediated TH regulation of TRH, distinct from regulation of MC4R neurons on the PVN, also separating the roles of mediating TH feedback and energy regulation. ⁹

The mapping of TSH signaling pathways in tanycytes indicates the heterogeneity of expression of transporters and deiodinases that may mediate the variable impact of NTI and other factors on regulation of the HPT axis. Furthermore, the TSH-dependent regulation of MAPK activity in tanycytes may affect tanycytic leptin transport, influencing feeding behavior and energy homeostasis. ¹⁰ Although GLP-1 agonists were not shown to directly regulate hypothalamic TRH, the effects of TH status on body weight remain an important clinical manifestation of hyperthyroidism and hypothyroidism.

The clinical implications of TH action in the hypothalamus include the thyroid function test changes observed in NTI, TH analogs for treatment of cardiometabolic diseases, and treatment of rarer conditions, such as patients with RTH-alpha. Perhaps most significantly these studies may elucidate some of the mechanisms underlying the recognition that levothyroxine monotherapy is associated with persistent symptoms among some hypothyroid patients, most prominently complaints of persistent weight gain.

The recognition of TR isoform specificity, as well as variable expression of Dio2 and Dio3 and thyroid transporters in tanycytes, may underlie differences in response to T4 and T3 in hypothyroidism. These findings should serve as a foundation for further investigations aimed at mapping the regulation of these important hypothalamic signaling pathways, offering both mechanistic insights and important clinical implications.