Elizabeth A. McAninch

Thyroid hormone signaling in energy homeostasis and energy metabolism

The thyroid hormone (TH) plays a significant role in diverse processes related to growth, development, differentiation, and metabolism. TH signaling modulates energy expenditure through both central and peripheral pathways. At the cellular level, the TH exerts its effects after concerted mechanisms facilitate binding to the TH receptor. In the hypothalamus, signals from a range of metabolic pathways, including appetite, temperature, afferent stimuli via the autonomic nervous system, availability of energy substrates, hormones, and other biologically active molecules, converge to maintain plasma TH at the appropriate level to preserve energy homeostasis. At the tissue level, TH actions on metabolism are controlled by transmembrane transporters, deiodinases, and TH receptors. In the modern environment, humans are susceptible to an energy surplus, which has resulted in an obesity epidemic and, thus, understanding the contribution of the TH to cellular and organism metabolism is increasingly relevant.

The role of thyroid hormone and brown adipose tissue in energy homoeostasis

The presence of brown adipose tissue (BAT) in adults has become increasingly well defined as a result of functional imaging studies of thermogenically active BAT. Findings from these studies have created a surge of scientific interest in BAT, because it represents a potential therapeutic target for obesity--a condition with profound health consequences and few successful therapies. BAT contributes to overall energy expenditure in small mammals and neonates through adaptive thermogenesis. Thyroid-hormone signalling, particularly through induction of type II deiodinase, has a central role in brown adipogenesis in vitro and BAT development in mouse embryos. Additionally, because of high intracellular expression of type II deiodinase, adult BAT has enhanced thyroid-hormone signalling with several thyroid-hormone-dependent thermogenic pathways, including expression of the genes Ppargc1a and Ucp1. BAT thermogenesis explains the essential part played by thyroid hormone in energy homoeostasis and adaptation to cold. Stimulation of BAT in adults, specifically through thyroid-hormone-mediated pathways, is a promising therapeutic target for obesity.

Prevalent polymorphism in thyroid hormone-activating enzyme leaves a genetic fingerprint that underlies associated clinical syndromes

CONTEXT A common polymorphism in the gene encoding the activating deiodinase (Thr92Ala-D2) is known to be associated with quality of life in millions of patients with hypothyroidism and with several organ-specific conditions. This polymorphism results in a single amino acid change within the D2 molecule where its susceptibility to ubiquitination and proteasomal degradation is regulated. OBJECTIVE To define the molecular mechanisms underlying associated conditions in carriers of the Thr92Ala-D2 polymorphism. DESIGN, SETTING, PATIENTS Microarray analyses of 19 postmortem human cerebral cortex samples were performed to establish a foundation for molecular studies via a cell model of HEK-293 cells stably expressing Thr92 or Ala92 D2. RESULTS The cerebral cortex of Thr92Ala-D2 carriers exhibits a transcriptional fingerprint that includes sets of genes involved in CNS diseases, ubiquitin, mitochondrial dysfunction (chromosomal genes encoding mitochondrial proteins), inflammation, apoptosis, DNA repair, and growth factor signaling. Similar findings were made in Ala92-D2-expressing HEK-293 cells and in both cases there was no evidence that thyroid hormone signaling was affected ie, the expression level of T3-responsive genes was unchanged, but that several other genes were differentially regulated. The combined microarray analyses (brain/cells) led to the development of an 81-gene classifier that correctly predicts the genotype of homozygous brain samples. In contrast to Thr92-D2, Ala92-D2 exhibits longer half-life and was consistently found in the Golgi. A number of Golgi-related genes were down-regulated in Ala92-D2-expressing cells, but were normalized after 24-h-treatment with the antioxidant N-acetylecysteine. CONCLUSIONS Ala92-D2 accumulates in the Golgi, where its presence and/or ensuing oxidative stress disrupts basic cellular functions and increases pre-apoptosis. These findings are reminiscent to disease mechanisms observed in other neurodegenerative disorders such as Huntingtons disease, and could contribute to the unresolved neurocognitive symptoms of affected carriers.

Scope and limitations of iodothyronine deiodinases in hypothyroidism

The coordinated expression and activity of the iodothyronine deiodinases regulate thyroid hormone levels in hypothyroidism. Once heralded as the pathway underpinning adequate thyroid-hormone replacement therapy with levothyroxine, the role of these enzymes has come into question as they have been implicated in both an inability to normalize serum levels of tri-iodothyronine (T3) and the incomplete resolution of hypothyroid symptoms. These observations, some of which were validated in animal models of levothyroxine monotherapy, challenge the paradigm that tissue levels of T3 and thyroid-hormone signalling can be fully restored by administration of levothyroxine alone. The low serum levels of T3 observed among patients receiving levothyroxine monotherapy occur as a consequence of type 2 iodothyronine deiodinase (DIO2) in the hypothalamus being fairly insensitive to ubiquitination. In addition, residual symptoms of hypothyroidism have been linked to a prevalent polymorphism in the DIO2 gene that might be a risk factor for neurodegenerative disease. Here, we discuss how these novel findings underscore the clinical importance of iodothyronine deiodinases in hypothyroidism and how an improved understanding of these enzymes might translate to therapeutic advances in the care of millions of patients with this condition.

Tissue-Specific Inactivation of Type 2 Deiodinase Reveals Multilevel Control of Fatty Acid Oxidation by Thyroid Hormone in the Mouse

Type 2 deiodinase (D2) converts the prohormone thyroxine (T4) to the metabolically active molecule 3,5,3′-triiodothyronine (T3), but its global inactivation unexpectedly lowers the respiratory exchange rate (respiratory quotient [RQ]) and decreases food intake. Here we used FloxD2 mice to generate systemically euthyroid fat-specific (FAT), astrocyte-specific (ASTRO), or skeletal-muscle-specific (SKM) D2 knockout (D2KO) mice that were monitored continuously. The ASTRO-D2KO mice also exhibited lower diurnal RQ and greater contribution of fatty acid oxidation to energy expenditure, but no differences in food intake were observed. In contrast, the FAT-D2KO mouse exhibited sustained (24 h) increase in RQ values, increased food intake, tolerance to glucose, and sensitivity to insulin, all supporting greater contribution of carbohydrate oxidation to energy expenditure. Furthermore, FAT-D2KO animals that were kept on a high-fat diet for 8 weeks gained more body weight and fat, indicating impaired brown adipose tissue (BAT) thermogenesis and/or inability to oxidize the fat excess. Acclimatization of FAT-D2KO mice at thermoneutrality dissipated both features of this phenotype. Muscle D2 does not seem to play a significant metabolic role given that SKM-D2KO animals exhibited no phenotype. The present findings are unique in that they were obtained in systemically euthyroid animals, revealing that brain D2 plays a dominant albeit indirect role in fatty acid oxidation via its sympathetic control of BAT activity. D2-generated T3 in BAT accelerates fatty acid oxidation and protects against diet-induced obesity.

Epicardial adipose tissue has a unique transcriptome modified in severe coronary artery disease

To explore the transcriptome of epicardial adipose tissue (EAT) as compared to subcutaneous adipose tissue (SAT) and its modifications in a small number of patients with coronary artery disease (CAD) versus valvulopathy.

The History and Future of Treatment of Hypothyroidism

Major diagnostic and therapeutic advancements in the early 20th century dramatically changed the prognosis of hypothyroidism from a highly morbid condition to one that could be successfully managed with safe, effective therapies. These advancements dictated treatment trends that have led to the adoption of L-thyroxine monotherapy, administered at doses to normalize serum thyroid-stimulating hormone (TSH), as the contemporary standard of care (Figure). Most patients do well with this approach, which both normalizes serum TSH levels and leads to symptomatic remission (1). Figure. Events influencing the evolution of treatment trends in hypothyroidism. Initial strategies for thyroid hormone replacement included thyroid transplantation, but efficacious pharmacologic strategies soon won favor. Natural thyroid preparations containing T4 and T3, such as desiccated thyroid, thyroid extracts, or thyroglobulin, were the initial pharmacologic agents. Synthetic agents were synthesized later. Early clinical trials demonstrated the efficacy of synthetic and natural agents, but concerns arose regarding consistency of natural thyroid preparations and adverse effects associated with T3-containing preparations (natural or synthetic). With the demonstration of peripheral T4-to-T3 conversion and the availability of the serum TSH radioimmunoassay in the early 1970s, there was a major trend in prescribing preference toward L-thyroxine monotherapy. BMR = basal metabolic rate; DT = desiccated thyroid; IV = intravenous; RIA = radioimmunoassay; T3 = triiodothyronine; T4 = thyroxine; TG = thyroglobulin; TSH = thyroid-stimulating hormone. Despite these successes, authors have questioned the efficacy of L-thyroxine monotherapy because about 10% to 15% of patients are dissatisfied as a result of residual symptoms of hypothyroidism (1, 2), including neurocognitive impairment (3), and about 15% of patients do not achieve normal serum triiodothyronine (T3) levels (4). Studies of several animal models indicate that maintaining normal serum T3 levels is a biological priority (5). Although the clinical significance of relatively low serum T3 in humans is not well-defined (1), evidence shows that elevating serum T3 through the administration of both L-thyroxine and L-triiodothyronine has benefited some patients (6, 7). However, this has not been consistently demonstrated across trials (1). Novel findings highlight the molecular mechanisms underlying the inability of L-thyroxine monotherapy to universally normalize measures of thyroid hormone signaling (8, 9), and new evidence may lay the foundation for a role of personalized medicine (10). Understanding the historical rationale for the trend toward L-thyroxine monotherapy allows us to identify scientific and clinical targets for future trials. Supplement. Original Version (PDF) Establishing the Need for Thyroid Replacement Cases of myxedema were reported in the mid19th century but were not initially connected with a deficiency from the thyroid gland until surgeons identified incident myxedema after thyroidectomy (11). Initial treatment strategies were largely insufficient and primarily symptom directed, including hot baths and institutionalization (12). The significant morbidity and mortality in the absence of efficacious treatment were clear, and thus the need to replace the thyroid through surgical transplantation or oral or intravenous routes was established. Thyroid transplant had some early successes, but for many patients symptoms recurred and the procedure even had to be repeated (13). Because of the rapidity and transiency of improvement (12), it was hypothesized that symptoms improved by absorption of the juice of the donor gland (14). Trials of the first pharmacologic strategies included intravenous or subcutaneous (12) or oral (15) administration of thyroid extract, in addition to thyroid feeding, the consumption of raw or cooked thyroid gland (16), with sustainable successes. Oral replacement strategies quickly won favor, although alarming symptoms associated with treatment were noted; however, the details were not fully described (17). Thyroid transplant may one day reemerge as a viable treatment option given that functional thyroid tissue can be generated from stem cells (18). Role of Basal Metabolic Rate and Serum Protein-Bound Iodine in Diagnosis and Treatment The association between hypothyroidism and energy expenditure was suspected clinically, and the discovery of lower O2 consumption in myxedema provided an early diagnostic tool (19). The development of a device to assess energy expenditure through measurement of the basal metabolic rate (BMR) in humans proved to be useful for not only diagnosis but also titration of therapy (20). The scale was calibrated so that a normal BMR reference range would be around 0%, whereas athyreotic individuals could have a BMR of about 40% (21). Because of lack of specificity (for example, low BMR in malnutrition), BMR was used in conjunction with the overall clinical impression; a low BMR in the setting of high clinical suspicion would secure a diagnosis and justify treatment (21, 22). L-Thyroxine was the first synthetic molecule used to treat hypothyroidism (23) and was shown to be efficacious as monotherapy for myxedema (24). Around that time, serum protein-bound iodine (PBI) emerged as a diagnostic test and therapeutic marker; serum PBI quantitation was the only valid way to biochemically assess thyroid hormone status (25). This tool was limited in terms of treatment monitoring because the effect on serum PBI varied by agent (26). For example, L-triiodothyronine corrected BMR without much increase in serum PBI, L-thyroxine increased serum PBI sometimes to above normal, and combination L-thyroxine and L-triiodothyronine and desiccated thyroid had the advantage of normalizing serum PBI (27). In addition to BMR and serum PBI, other surrogates for treatment response included cholesterol levels, symptoms, and deep tendon reflexes, but their lack of sensitivity was always recognized (28). Evidence of Overtreatment in Early Trials With the availability of multiple forms of thyroid hormone replacement, early clinical trials were designed to assess efficacy and dose equivalency among natural thyroid (typically desiccated), synthetic L-thyroxine, and/or L-triiodothyronine. These were not designed as superiority trials, their therapeutic goals were the normalization of serum PBI or BMR, and doses were dramatically higher than used today. For example, desiccated thyroid and intravenous L-thyroxine monotherapy normalized BMR, pulse, and body weight in myxedema (29), L-triiodothyronine monotherapy was likewise effective (30), and the potency of L-triiodothyronine exceeded that of L-thyroxine (31). These clinical trials also began to define the adverse-effect profiles associated with these agents; thyrotoxicosis was frequently encountered. Patients treated with L-triiodothyronine3 (100 to 175 mcg/d) normalized BMR faster than did those receiving desiccated thyroid (120 to 210 mg/d) or L-thyroxine (200 to 350 mcg/d) but were more likely to experience angina (32). Desiccated thyroid was also associated with adverse symptoms in other studies; muscle stiffness, psychosis, and angina all occurred (33). In a crossover study of L-triiodothyronine monotherapy (75 to 100 mcg/d), L-thyroxine monotherapy (200 to 300 mcg/d), and desiccated thyroid (1.5 to 3 grains/d), all of these therapies restored BMR and serum PBI; with L-triiodothyronine, however, angina and heart failure occurred. Dose reduction corrected these adverse effects, but authors concluded that L-thyroxine monotherapy or thyroid extract was preferred (34). In a trial of L-thyroxine monotherapy at doses of 200 to 300 mcg/d versus L-thyroxine (80 mcg) plus L-triiodothyronine (20 mcg) daily, patients receiving the combination had such symptoms as palpitations, nervousness, tremor, and perspiration (35). Some early proponents of L-thyroxine monotherapy emerged because of less frequent thyrotoxic effects (24), but it is difficult to determine whether such adverse effects were related to the agent used or its high dosage. Thyrotoxic adverse effects were typically remediable by simple dose reduction (36), so desiccated thyroid remained the preparation of choice (37). Rise and Fall of Natural Thyroid Products From the early 1890s through the mid-1970s, desiccated thyroid was the preferred form of therapy for hypothyroidism (Appendix Table). This preference was reinforced by the unique ability of desiccated thyroid to reproduce a normal serum PBI (33). The predominance of natural thyroid products was illustrated by prescribing patterns in the United States: In 1965, approximately 4 of every 5 prescriptions for thyroid hormone were for natural thyroid preparations (38). Concerns about inconsistencies in the potency of these tablets arose (26) after the discovery that some contained anywhere from double to no detectable metabolic activity (39). The shelf-life of desiccated tablets was limited, especially if the tablets were kept in humid conditions (36). There were reports of patients not responding to desiccated thyroid altogether because their tablets contained no active thyroid hormone. It was not until 1985 that the revision of the U.S. Pharmacopeia standard from iodine content to T3/thyroxine (T4) content resulted in stable potency (38), but by then the reputation of natural thyroid products was tarnished (40). Appendix Table. Trends in the Treatment Recommendations for Hypothyroidism* Physicians hesitated to use L-thyroxine monotherapy over concern that it could result in a relative T3 deficiency, despite growing discontent with potency of natural thyroid products (39) and reduced cost of L-thyroxine, such that the 2 treatments were approximately equivalent (36, 41). The seminal discovery of peripheral T4-to-T3 conversion in athyreotic individuals largely obviated this concern (42). This laid the foundation

Type 2 Deiodinase Disruption in Astrocytes Results in Anxiety-Depressive-Like Behavior in Male Mice

Millions of levothyroxine-treated hypothyroid patients complain of impaired cognition despite normal TSH serum levels. This could reflect abnormalities in the type 2 deiodinase (D2)-mediated T4-to-T3 conversion, given their much greater dependence on the D2 pathway for T3 production. T3 normally reaches the brain directly from the circulation or is produced locally by D2 in astrocytes. Here we report that mice with astrocyte-specific Dio2 inactivation (Astro-D2KO) have normal serum T3 but exhibit anxiety-depression-like behavior as found in open field and elevated plus maze studies and when tested for depression using the tail-suspension and the forced-swimming tests. Remarkably, 4 weeks of daily treadmill exercise sessions eliminated this phenotype. Microarray gene expression profiling of the Astro-D2KO hippocampi identified an enrichment of three gene sets related to inflammation and impoverishment of three gene sets related to mitochondrial function and response to oxidative stress. Despite normal neurogenesis, the Astro-D2KO hippocampi exhibited decreased expression of four of six known to be positively regulated genes by T3, ie, Mbp (∼43%), Mag (∼34%), Hr (∼49%), and Aldh1a1 (∼61%) and increased expression of 3 of 12 genes negatively regulated by T3, ie, Dgkg (∼17%), Syce2 (∼26%), and Col6a1 (∼3-fold) by quantitative real-time PCR. Notably, in Astro-D2KO animals, there was also a reduction in mRNA levels of genes known to be affected in classical animal models of depression, ie, Bdnf (∼18%), Ntf3 (∼43%), Nmdar (∼26%), and GR (∼20%), which were also normalized by daily exercise sessions. These findings suggest that defects in Dio2 expression in the brain could result in mood and behavioral disorders.

New insights into the variable effectiveness of levothyroxine monotherapy for hypothyroidism

Thyroid hormone replacement has been the mainstay of treatments for hypothyroidism since the 19th century. Animal thyroid preparations, which contain thyroxine (T4) and tri-iodothyronine (T3), were the first pharmacotherapies, and synthetic agents—eg, levothyroxine (also known as LT4)—are the current standard of care.1 Chemical composition of hormone replacement therapy is important in view of the clinical data suggesting that levothyroxine monotherapy does not consistently normalise serum T3 concentrations1 or universally restore clinical euthyroidism. Although the clinical significance is not clear, increasing serum T3 with a combination of levothyroxine plus liothyronine (also known as LT3) results in weight loss and improves psychological function in some patients.1

Coccidiomycosis Thyroiditis in an Immunocompromised Host Post-Transplant: Case Report and Literature Review

CONTEXT Acute infectious thyroiditis, particularly fungal thyroiditis, is rare and typically presents in immunocompromised individuals. Here we report the first case of coccidiomycosis thyroiditis occurring in an organ recipient as a consequence of likely allograft contamination and discuss the management strategies for thyroid masses in the setting of disseminated infection. EVIDENCE ACQUISITION AND SYNTHESIS In this clinical case seminar, we summarize the previously published cases of coccidiomycosis thyroiditis based on a MEDLINE search of all peer-reviewed publications (original articles and reviews) on this topic. We identified six other cases, five of which also occurred in immunocompromised hosts, although none occurred in organ recipients. CONCLUSION A case of coccidiomycosis thyroiditis occurring in a post-liver transplant immunocompromised host is reported. Analysis of donor serum revealed the liver allograft as the likely infectious source, resulting in hematological spread to the thyroid. Although our patients thyroid gland was lacking gross structural abnormalities at presentation, new-onset thyroid masses developed after relative immune restoration and initiation of antifungal therapies. The differential diagnosis of new-onset thyroid masses in immunocompromised hosts is discussed, with a focus on immune reconstitution inflammatory syndrome. The role of thyroidectomy in the management of fungal thyroiditis is also discussed.