The role of thyroid hormone in metabolism and metabolic syndrome

Molecular mechanism of action of thyroid hormones: general overview

THs act on several target peripheral tissues via several mechanisms. Briefly, thyroxine (T4), which is the main product of the thyroid gland, is converted to the active hormone, T3, an enzymatic reaction catalyzed by type 1 (D1) or type 2 5′deiodinases (D2). T4 and T3 can be inactivated by type 3 5-deiodinase (D3). T4 and T3 enter cells through specific membrane transporters, and T3, originating from the circulation or from intracellular conversion of T4 to T3, binds to TH receptors, subtypes 1, β1 or β2, located at the nucleus to regulate the transcriptional activity of target genes. ¹¹⁸ This is the canonical pathway; however, recently, other non-classical pathways have been reported. TH actions may be mediated by cytoplasmic or mitochondrial TH receptors (TR), or through binding to unspecific membrane proteins that activate intracellular signaling cascades.^118–121 These non-canonical signaling pathways have been reported to be especially important to the cardiometabolic effects of thyroid hormones. ¹²¹ In that elegant study, the authors employed genetically manipulated mice to differentiate between T3 effects mediated by the canonical and non-canonical pathways. They showed that the acute hypoglycemic effect of T3 is dependent on TRβ but does not require deoxyribonucleic acid binding. Its action involves activation of the phosphatidylinositol 3-kinase (PI3K) signaling cascade. The same non-canonical signaling pathway is involved in a T3-lowering effect in serum and hepatic triglycerides. In addition, T3 actions in metabolic rate and energy expenditure, as well as in the exogenous control of heart rate have important contributions of the non-canonical signaling pathways. ¹²¹

It is also important to mention that tissue responsiveness to TH may vary with age and sex, which may be related to tissue-specific alterations in T4 to T3 conversion.^122–124 The interplay between age and sex are particularly interesting in TH-induced changes in body weight and energy expenditure in mice, with sex modifying the response of TH differently in old males compared with old females.^122–124

Thyroid hormones influencing adiposity

Adiposity gain or loss depends primarily on the balance between energy expenditure (EE) and energy intake (EI). Resting EE (REE) is solely used in the cellular process to maintain life. ¹³³ EE can be stimulated by physical activity or acceleration of different metabolic processes, resulting in heat production (facultative thermogenesis). ¹³⁴ The balance between EE and EI depends mainly on satiety control, sympathetic nervous system (SNS) activity, and the endocrine system. THs are strong regulators of the metabolic rate with consequent effects on different outcomes, including adiposity. ¹³⁵ However, as previously described, the relationship between TH and adiposity is bidirectional, since TH and also thyroid-stimulating hormone (TSH) levels have effects on adiposity, which in turn may act on thyroid function and perhaps on the structure of this gland.^136,137 Adiposity leads to production of several hormones, cytokines, and other compounds that influence thyroid function, as described in the next sections.

THs, especially T3 produced by enzymatic reaction catalyzed by type 1 (D1) or type 2 5′deiodinases (D2), are enrolled in controlling metabolic rate by several mechanisms, as explained in the following sections of this manuscript. In summary, they exert direct effects on adenosine triphosphate (ATP) utilization, uncoupling synthesis of ATP, mitochondrial biogenesis and have inotropic and chronotropic effects on body. THs also act controlling core body temperature, appetite, and sympathetic activity. Additionally to T4 and T3, other TH metabolites exert similar effects.^138,139 It has been demonstrated that 3,5 diiodo-L-thyronine (T2) prevents high-fat-diet-induced adiposity by means of increasing EE and promoting anti-adipogenic and anti-lipogenic pathways in white adipose tissue (WAT).^138,139 Also, studies have demonstrated that decarboxylated TH molecules, termed thyronamines, when given to animals, lead to metabolic effects that generally oppose the direction of T3. Thyronamines are primarily produced in the thyroid, but there is evidence that they may be produced in other tissues.^139–141 The physiological and clinical relevance of TH metabolites is under intense investigation.^139–141

The thermogenic effects of TH, especially T3, are well known, and hyperthyroid patients have an increase in heat production and are heat intolerant. Hyperthyroid patients are opposite to hypothyroid patients, who produce less heat and are cold intolerant. ¹⁴² After thyroid hormone administration there is an increase in oxygen consumption in most tissues. ¹⁴² THs cause a direct increase in adenosine triphosphate (ATP) utilization leading to acceleration of anabolic and catabolic pathways in the macronutrient metabolism, such as lipolysis/fatty-acid oxidation and increased protein turnover. ¹⁴³ In addition, THs stimulate the sodium/potassium (Na⁺/K⁺) ATPase and the sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA) that mediate ion transport through membranes, processes that require ATP utilization, leading to increasing of it consumption and contributing to thermogenesis. ¹⁴⁴ Therefore, thyroid hormone increased the utilization of energy reserves, such as lipids from the adipose tissue.

Another mechanism by which TH may increase the REE is related to the hormones’ inotropic and chronotropic effects, exerted in conjunction with the SNS, since it is well known that part of REE is related to cardiac function. ¹⁴⁵

TH actions at the mitochondria are very important in thermogenesis. In addition to promoting mitochondrial biogenesis, THs act to uncouple the synthesis of ATP from heat production in the mitochondria. ¹⁴² This uncoupling is mediated by their action on mitochondrial uncoupling proteins (UCP) that lead to non-shivering thermogenesis via conversion of chemical energy to heat without an increase in ATP production. The presence of this mechanism, in which promoting uncoupling phosphorylation in brown adipose tissue (BAT) is promoted, is one of the markers of evolutionary process of mammals; however, for many years it was thought that BAT was not present in adults. Nevertheless, in the past decade, the presence of active BAT in adult humans has been demonstrated and its amounts are inversely associated with body weight and serum glucose levels.^{146,147–152} The action of TH in this tissue gains attention as additional mechanisms enrolled in MetS.

In BAT, type 1 UCP (UCP1) is the hallmark of thermogenesis. This UCP expression is stimulated by T3, which is locally generated from T4 by intracellular D2. This D2 is positively regulated by beta-adrenergic activity. ¹⁵² THs cause an upregulation of adrenergic receptor expression, leading to an amplified effect on UCP1 expression, which is also activated by the SNS. ¹⁵² Studies have shown that D2 is very important to TH-induced adaptive type of thermogenesis in BAT. ¹⁵² D2 also responds to other thermogenic inductors, as highlighted by a recent study showing that the adipokine, adipocyte fatty-acid-binding protein (A-FABP), requires BAT D2 activity to exert its thermogenic effects. ¹⁵³

Another postulated effect of THs in BAT is the stimulation of WAT ‘browning,’ which consists of the acquisition of brown-fat characteristics by a certain group of WAT cells, termed beige cells. ¹⁵⁴ Although it would be an attractive tool in obesity treatment, evidence in humans is still scarce, ¹⁵² and a recent experimental study does not support that TH-induced browning is accompanied by an increase in thermogenesis. ¹⁵⁵ TH also stimulates the expression of other UCPs, such as UCP2 and 3, and the latter is very important to thermogenesis and fatty oxidation in muscle. ¹⁵⁶

In addition to acting on peripheral tissues, THs also have relevant modulatory actions in the central nervous system with respect to core body temperature, satiety control, and activity of the SNS. ¹⁵⁷ The action of T3 on the hypothalamus, more specifically on the ventromedial hypothalamus (VMH), stimulates the SNS that not only stimulates TH production but also acts in combination with THs in those same peripheral tissues that affect the MetS components.^125–127

Central T3 administration results in increased body temperature, concomitant with reduction of levels of hypothalamic AMP-activated protein kinase (AMPK), increased tone in the sympathetic nerves innervating BAT.^158,159 Hypothalamic AMPK and fatty-acid metabolism mediate thyroid regulation of energy balance.^158–160 Those responses involve UCP1, since they were abrogated in UCP1 knockout mice. ¹⁶¹

Hyperthyroid individuals frequently have hyperphagia even in the presence of weight lost, ¹⁵⁷ which is related in great part to the direct effect of THs on appetite stimulation. In the hypothalamic nucleus arcuate, T3, produced locally by D2, increases the expression of the orexigenic peptides neuropeptide Y (NPY) and agouti-related peptide (AgRP), and decreases the anorexigenic peptide, pro-opiomelanocortin (POMC), ¹⁶⁰ and the reverse events occur in hypothyroid rats. ¹⁶² Acting at the VMH, T3, in low doses, was shown to induce an increase in food intake and potently stimulate the sympathetic activity and BAT thermogenesis.^126,163,164 In contrast, Hameed and colleagues demonstrated that ablation of the β isoform of the TR only at the VMH of adult rats led to increase in AgRP/NPY and reduction in POMC pathways, with a concurrent augmentation in food intake and weight gain. ¹⁶⁵ This effect was not observed when both isoforms of TR had downregulated functions in the VMH. ¹⁶⁰ Therefore, not only the availability of T3, but also the specific TR isoform, determines the final effect of THs in control of hypothalamic circuits controlling energy homeostasis.

The action of TH in the regulation of EE may be indirect via controlling the action with or without expression of other circulating or local factors. Recently, it has been reported that irisin, a hormone produced in striate muscle after exercise, ¹⁶⁶ induces browning of WAT and shows a possible relation with thyroid function. ¹⁶⁷ However, human studies present conflicting results regarding the association between thyroid function and irisin levels, with some studies demonstrating higher levels in hyperthyroidism^168,169 and low levels in hypothyroid patients.^170–172 However, these results were not confirmed in all studies.^173–175

Altered thyroid function can modify circulating levels of fibroblast growth factor 21 (FGF21), fetuin A, and neuregulin 4 (NgL-4), among others, which modulate EE.^27,48 NgL-4 is an epidermal growth factor (EGF) family member that is secreted by BAT and promotes augmentation in EE, inhibition of hepatic lipogenesis, and reduction of fat-mass storage. ¹⁷⁶ A study with 129 hyperthyroid patients demonstrated that they had higher levels of NgL-4 than controls, which showed a reduction in these levels after restoring euthyroidism with treatment. ¹⁷⁷ Studies evaluating possible opposite effects, leading to reduction of NgL-4 in hypothyroidism, are still lacking.

In addition to TH, TSH has been shown to act directly in adipose tissue that expresses TSH receptors. In differentiated human adipocytes, TSH induces lipolysis and inhibits insulin signaling through protein kinase B (Akt) phosphorylation, ¹⁷⁸ which might contribute to IR. However, Ma and coworkers showed that TSH appears to stimulate adipocyte differentiation and lipogenesis in the pre-adipocyte cell lineage 3T3-L1 through a mechanism involving peroxisome-proliferated-activator–receptor (PPAR) gamma. ¹⁷⁹ In agreement with a role of TSH as an adipogenic factor, mice that did not express the TSH receptor and were under TH supplementation, exhibited resistance to high-fat-diet-induced obesity. ¹⁷⁹

Adiposity influencing thyroid function

Leptin is a hormone produced by adipose tissue in direct proportion to the quantity of adipose tissue mass. Leptin acts mainly at hypothalamic neurons to induce satiety and increase EE. Patients with genetic mutations in the leptin gene or leptin receptor are obese, and chronic reposition of leptin caused normalization of their body weight. However, most obese patients have hyperleptinemia but are resistant to the anorexigenic central action of leptin.^180,181

In addition, leptin was shown to regulate the production of neurohormones in the medio-basal hypothalamus, among them, thyrotropin-releasing hormone (TRH) neurons of the periventricular nucleus.^181,182 In another study, leptin activated TRH neurons both directly and indirectly, acting through the Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway.^182,183 The increase in TRH release was shown to lead to higher pituitary secretion of TSH,^182–184 which in turn, stimulates thyroid function and proliferation.

Besides acting as a stimulatory agent for TRH secretion, the overall response of the thyroid axis to leptin is controversial among species and depends on nutritional status. ¹⁸⁵ Both rodents and humans subject to fasting show suppression of TH function, with concomitant decreases in serum levels of leptin, and replacement of leptin partially restored normal concentrations of thyroid hormones.^186–189 Therefore, during caloric deprivation, the reduction in leptin seems to contribute to an integrated response to fasting, including thyroid-function suppression. However, in conditions with hyperleptinemia or at physiological levels, the role of leptin in thyroid function is less clear and may also reflect other leptin actions in the pituitary, thyroid, and peripheral tissues. Leptin receptors have been found in the anterior pituitary and thyroid gland, and direct inhibitory actions on TSH secretion and on the expressions of the Na⁺/I^– symporter (NIS) and thyroglobulin messenger ribonucleic acid (mRNA) in thyroid cell lines have been reported.^184,190 Additionally, there is experimental evidence from rodent studies that thyroid hormone metabolism may be modulated by leptin. Exogenous leptin administration caused an increase in D1 activity in the liver and pituitary, while causing a reduction in D2 activity at the hypothalamus and in BAT. Therefore, leptin may modulate thyroid hormone actions in target tissues, but collectively, these studies indicate that nutritional status and thyroid state clearly modify the responses to leptin.^191–194

Another postulated mechanism of the way in which obesity is related to thyroid disfunction concerns chronic low-grade inflammation in adipose tissue that secretes cytokines and may affect thyroid function. It has been demonstrated that tumor necrosis factor alpha (TNF-α) and interleukins 1 and 6 (IL-1 and -6) inhibit the mRNA expression of the NIS. ¹⁹⁵ Additionally, pro-inflammatory cytokines have been associated with inhibition of D1 in HepG2 hepatocarcinoma cells ¹⁹⁶ and induction of D3, ¹⁹⁷ resulting in a decrease in serum T3, one feature of the low T3 syndrome associated with chronic diseases. ¹⁹⁸

Finally, IR, in conjunction with leptin levels, appears to be related to obesity and leads to augmentation of serum TSH levels.^199,200 Recent studies give support to this hypothesis, showing that metformin, a drug used to improve insulin sensitivity, may cause a reduction in serum TSH levels.^201,202 Different mechanisms have been proposed and the activation of the AMP-activated protein kinase (AMPK) pathway may be enrolled.^{158,159,203,204}

Thyroid function acting on glucose metabolism

Hypothyroidism is associated with peripheral IR due to a reduction in glucose uptake, and on the other hand, hyperthyroidism increases glycemia due to an increase in liver production.^205–207 T3 acts directly on the liver through TRβ, regulating genes involved in hepatic gluconeogenesis, glycogen metabolism, and insulin signaling.^205,206 In addition, TH also acts centrally on the hypothalamus to increase sympathetic flow to the liver. ¹²⁶ As a consequence, in the liver, there is a decrease in glycogen synthesis and increase in gluconeogenesis and glucogenolysis,^126,207 leading to an increase in glucose output. ²⁰⁸ T3 increases the translocation of the glucose transport 4 (GLUT 4) to the plasma membrane in skeletal muscle and adipose tissue, which is associated with better glucose tolerance.^208–215 T2 administration has also been associated with better glucose tolerance in animal models. It induces inhibition of hepatic gluconeogenesis gene expression^216–218 by means of modulation of microRNA, ²¹⁷ and regulation of the activity of the protein kinase mammalian target of rapamycin complexes 1 (mTORC1) and 2 (mTORC2). ²¹⁸

Although THs play a role in islet trophic state maintenance, ²¹⁹ hyperthyroidism impairs glucose-stimulated insulin secretion and accelerates insulin degradation. ²²⁰ In the insulin-producing cell line, INS-1 cells, at high concentrations, T3 induced B-cell apoptosis and death. ²²¹ Also, T2, at high concentrations, is able to decrease the glucose-induced insulin secretion, even though both T2 and T3 have a stimulatory effect at low concentrations. ²²² The importance of maintaining low levels of T3 in pancreatic β cells was shown in mice with specific β-cell pancreatic deletion of D3 that showed a decrease in pancreatic islet area, insulin-gene expression, and glucose-stimulated insulin secretion, even though the mice were euthyroid. ²²³

Thyroid function acting on lipid metabolism related to metabolic syndrome

The lipid abnormalities related to MetS are hypertriglyceridemia and low serum HDL-c levels. These abnormalities will be the focus of the present revision despite a high number of studies evaluating several other alterations in lipid profile associated with thyroid function.^224,225

THs have effects throughout the whole body, stimulating both lipid synthesis and degradation, but in the hyperthyroid condition, there is a predominant increase in lipolysis from fat stores. ¹⁴² In the liver, THs stimulate the re-esterification of free fatty acids into triacylglycerol and also induce de novo lipogenesis from glucose metabolism. ²²⁶ However, THs also concurrently stimulate fatty-acid oxidation, and, under physiological conditions, the result is a balance that does not increase hepatic triacylglycerol levels. ²²⁶ The mechanisms of TH action involve direct regulation of the transcription rate of specific lipogenic/oxidative genes, in addition to alterations in the concentrations of metabolites, energy state of the cells, and post-translational modifications of proteins involved in the liver lipid metabolism.^117,226

TH increases cholesterol clearance because even though they stimulate endogenous cholesterol synthesis, they potently increase hepatic cholesterol uptake and excretion as bile acids. ²²⁷ Low-density lipoprotein (LDL)-c accumulates in the serum of hypothyroid patients since the LDL-receptor and the sterol regulatory element-binding protein 2 (SREBP2) are under-expressed in hypothyroidism. LDL-receptors mediate liver uptake of cholesterol that comes from peripheral tissues. SREBP2 is a key transcription factor that induces the expression of lipogenic-related genes, including Ldlr. ²²⁷ Levels of very-low-density lipoprotein (VLDL) in the liver and in serum are influenced by lipoprotein lipases that are up-regulated by thyroid hormones, a mechanism that may contribute to the high serum triglycerides in hypothyroidism. ²²⁸ In addition, ApoB100 levels are reduced by THs contributing to the increase in VLDL and LDL production observed in the liver during hypothyroidism. ²²⁹

An increase in serum HDL-c has been reported in hypothyroid patients; this finding appears to be related to a decrease in activity of the cholesterol ester transfer protein (CEPT). ²²⁸ CEPT, which is positively regulated by THs, mediates the exchange of cholesteryl-ester between HDL-c and VLDL and also has a pro-atherogenic role. Higher expression of CEPT would lead to higher cardiovascular risk, related to augmentation of serum levels of VLDL and reduction of HDL-c. However, as serum levels of HLD-c are also influenced by several other mechanisms, and are reduced in states of IR and obesity, there are disagreements with respect to the results of human studies regarding thyroid function and serum HDL-c, as shown in Table 2. HDL-c levels in hypothyroid patients might also be reduced when obesity diagnosis is present with marked reduction of insulin sensitivity or MetS.

Additionally, administration of T2 in rodents has hypolipemic action, affecting the hepatic lipid metabolism. ¹²⁹ It has been demonstrated that T2 is able to increase hepatic lipid oxidation and contrary to T3, does not stimulate the lipogenic pathway in animals fed a high-fat diet, ²³⁰ which potentially contributes to the important effect reported in avoiding lipid accumulation in the liver of those animals. Despite the evidence in rodents, the physiological role of T2 in human metabolism, and potential therapeutic use, need further clarification.^231,232 Serum levels of 3,5-T2 have been associated with several clinical conditions, like impaired renal function, sepsis, and oral LT4 (levothyroxine) supplementation; ²³² however, further studies are necessary to evaluate causative effects between the found associations. These studies may benefit from a recently developed method to measure 3,5-T2 in human serum by mass spectrometry, which, interestingly, showed correlation with T2 isomer 3,3'-T2, but not with serum T3 or T4. ²³³ Likewise, other methods to measure 3,5-T2 by mass spectrometry have been tested.^234–236

Thyroid hormone acting on blood pressure

THs act on the vasculature and in the heart by TR-mediated gene regulation in the nucleus and also via other non-classical pathways at the cytoplasmatic and cellular membrane levels.^130,237

In myocytes, and also in vasculature, THs, especially T3 with greater affinity, bind to TH nuclear receptors in its two isoforms, TRα and TRβ. Thereafter, the complex formed by TH response elements at the promoter regions of specific responsive genes lead to positive or negative regulation of several genes enrolled in cardiac function and vascular resistance. The sarcoplasmic reticulum calcium ATPase (SERCA2), the myosine-have chains-α (αMHC), the Na⁺/K⁺ ATPase, the voltage-gated K⁺ channels, the adenine nucleotide translocase (ANT1) and the β-adrenergic receptor are positively regulated by THs. In opposite, the myosine-have chains-β (βMHC), the phospholamban, the Na⁺/Ca²⁺ exchanger (NCX1), the TRα1, adenylyl cyclase (types V, VI) and TH transporters 8 and 10 are negatively regulated by THs.^130,237

Additionally to genomic effects of TH on cardiac myocytes, and also on vasculature, there are important and faster non-genomic actions, like those related to direct modulation of membrane ion channels. ¹³⁰

THs have important inotropic and chronotropic effects on the heart and concomitantly, they cause vasodilatation in the systemic circulation, leading to a decrease in systemic vascular resistance. Hyperthyroid patients exhibit tachycardia, increased heart contractility, and decreased cardiac after-load, resulting in increased cardiac output, which leads to systolic hypertension. Hypothyroid patients may exhibit diastolic hypertension, associated with impaired endothelial-dependent vasodilatation. ²³⁸ Alterations in the microcirculation of hypothyroid patients have also been reported, such as a decrease in blood-flow velocity and impaired vasodilation after a short period of ischemia. ²³⁹ The mechanism involves TH stimulation of nitric oxide production and regulation of other local regulatory factors, resulting in a decrease in vascular smooth muscular tone.^239–242

In addition, TH actions in the central nervous system have an influence on autonomic regulation of BP. Recently, a group of parvalbuminergic neurons at the anterior hypothalamus, which act to decrease BP, was described, and their development appears to be dependent on TRα signaling. ²⁴³ This finding may explain the hypotension present in patients with TRα mutations. ²⁴⁴ Different from peripheral systemic vasculature, the pulmonary vasculature does not respond to the vasodilator effect of TH and may explain reversible pulmonary hypertension related to hyperthyroidism. ²⁴⁵