Successful treatment of hyperthyroidism due to nonneoplastic pituitary TSH hypersecretion with 3,5,3'-triiodothyroacetic acid (TRIAC)

Molecular Functions of Thyroid Hormone Signaling in Regulation of Cancer Progression and Anti-Apoptosis

Several physiological processes, including cellular growth, embryonic development, differentiation, metabolism and proliferation, are modulated by genomic and nongenomic actions of thyroid hormones (TH). Several intracellular and extracellular candidate proteins are regulated by THs. 3,3,5-Triiodo-L-thyronine (T₃) can interact with nuclear thyroid hormone receptors (TR) to modulate transcriptional activities via thyroid hormone response elements (TRE) in the regulatory regions of target genes or bind receptor molecules showing no structural homology to TRs, such as the cell surface receptor site on integrin αvβ3. Additionally, L-thyroxine (T₄) binding to integrin αvβ3 is reported to induce gene expression through initiating non-genomic actions, further influencing angiogenesis and cell proliferation. Notably, thyroid hormones not only regulate the physiological processes of normal cells but also stimulate cancer cell proliferation via dysregulation of molecular and signaling pathways. Clinical hypothyroidism is associated with delayed cancer growth. Conversely, hyperthyroidism is correlated with cancer prevalence in various tumor types, including breast, thyroid, lung, brain, liver and colorectal cancer. In specific types of cancer, both nuclear thyroid hormone receptor isoforms and those on the extracellular domain of integrin αvβ3 are high risk factors and considered potential therapeutic targets. In addition, thyroid hormone analogs showing substantial thyromimetic activity, including triiodothyroacetic acid (Triac), an acetic acid metabolite of T₃, and tetraiodothyroacetic acid (Tetrac), a derivative of T₄, have been shown to reduce risk of cancer progression, enhance therapeutic effects and suppress cancer recurrence. Here, we have reviewed recent studies focusing on the roles of THs and TRs in five cancer types and further discussed the potential therapeutic applications and underlying molecular mechanisms of THs.

The Colorful Diversity of Thyroid Hormone Metabolites

Since the discovery of L-thyroxine, the main secretory product of the thyroid gland, and its major metabolite T3, which exerts the majority of thyroid hormone action via ligand-dependent modulation of the function of T3 receptors in nuclei, mitochondria, and other subcellular compartments, various other T4-derived endogenous metabolites have been identified in blood and tissues of humans, animals, and early protochordates. This review addresses major historical milestones and experimental findings resulting in the discovery of the key enzymes of thyroid hormone metabolism, the three selenoprotein deiodinases, as well as the decarboxylases and amine oxidases involved in formation and degradation of recently identified endogenous thyroid hormone metabolites, i.e. 3-iodothyronamine and 3-thyroacetic acid. The concerted action of deiodinases 2 and 3 in regulation of local T3 availability is discussed. Special attention is given to the role of the thyromimetic "hot" metabolite 3,5-T2 and the "cool" 3-iodothyronamine, especially after administration of pharmacological doses of these endogenous thyroid hormone metabolites in various animal experimental models. In addition, available information on the biological roles of the two major acetic acid derivatives of thyroid hormones, i.e. Tetrac and Triac, as well as sulfated metabolites of thyroid hormones is reviewed. This review addresses the consequences of the existence of this broad spectrum of endogenous thyroid hormone metabolites, the "thyronome," beyond the classical thyroid hormone profile comprising T4, T3, and rT3 for appropriate analytical coverage and clinical diagnostics using mass spectrometry versus immunoassays for determination of total and free concentrations of thyroid hormone metabolites in blood and tissues.

Syndromes of Resistance to Thyroid Hormone Action

Thyroid hormone (TH) action is crucial for the development of several tissues.A number of syndromes are associated with reduced responsiveness to thyroid hormones, expanding the original definition of thyroid hormone resistance, firstly described by Refetoff and collaborators in 1967, which is characterized by elevated circulating levels of T4 and T3 with measurable serum TSH concentrations, as a consequence of mutations of thyroid hormone receptor beta (TRβ), recently named as RTHβ. More recently, another form of insensitivity to TH has been identified due to mutations in the thyroid hormone receptor alpha (TRα), named RTHα. In this chapter we will focus the discussion on the phenotype of RTHβ and RTHα. These diseases share the same pathogenic mechanism caused by dominant negative mutations in TH receptor genes that reduce T3 binding or affect the recruitment of cofactors. As a consequence, thyroid hormone actions are impaired at the tissue level. The phenotypic manifestations of RTHβ and RTHα are to some extent correlated with the degree of disruption and the tissue distribution of the TRs being characterized by variable coexistence of hypothyroid or thyrotoxic manifestations in RTHβ or by a congenital hypothyroid features in RTHα despite normal TSH and borderline low free T4.

From zebrafish to human: A comparative approach to elucidate the role of the thyroid hormone transporter MCT8 during brain development

Monocarboxylate transporter 8 (MCT8) facilitates transmembrane transport of thyroid hormones (THs) ensuring their action on gene expression during vertebrate neurodevelopment. A loss of MCT8 in humans results in severe psychomotor deficits associated with the Allan-Herndon-Dudley Syndrome (AHDS). However, where and when exactly a lack of MCT8 causes the neurological manifestations remains unclear because of the varying expression pattern of MCT8 between specific brain regions and cells. Here, we elaborate on the animal models that have been generated to elucidate the mechanisms underlying MCT8-deficient brain development. The absence of a clear neurological phenotype in Mct8 knockout mice made it clear that a single species would not suffice. The evolutionary conservation of TH action on neurodevelopment as well as the components regulating TH signalling however offers the opportunity to answer different aspects of MCT8 function in brain development using different vertebrate species. Moreover, the plethora of tools for genome editing available today facilitates gene silencing in these animals as well. Studies in the recently generated mct8-deficient zebrafish and Mct8/Oatp1c1 double knockout mice have put forward the current paradigm of impaired TH uptake at the level of the blood-brain barrier during peri- and postnatal development as being the main pathophysiological mechanism of AHDS. RNAi vector-based, cell-specific induction of MCT8 knockdown in the chicken embryo points to an additional function of MCT8 at the level of the neural progenitors during early brain development. Future studies including also additional in vivo models like Xenopus or in vitro approaches such as induced pluripotent stem cells will continue to help unravelling the exact role of MCT8 in developmental events. In the end, this multispecies approach will lead to a unifying thesis regarding the cellular and molecular mechanisms responsible for the neurological phenotype in AHDS patients.

Therapeutic applications of thyroid hormone analogues in resistance to thyroid hormone (RTH) syndromes

Thyroid hormone (TH) is crucial for normal development and metabolism of virtually all tissues. TH signaling is predominantly mediated through binding of the bioactive hormone 3,3',5-triiodothyronine (T3) to the nuclear T3-receptors (TRs). The intracellular TH levels are importantly regulated by transporter proteins that facilitate the transport of TH across the cell membrane and by the three deiodinating enzymes. Defects at the level of the TRs, deiodinases and transporter proteins result in resistance to thyroid hormone (RTH) syndromes. Compounds with thyromimetic potency but with different (bio)chemical properties compared to T3 may hold therapeutic potential in these syndromes by bypassing defective transporters or binding to mutant TRs. Such TH analogues have the potential to rescue TH signaling. This review describes the role of TH analogues in the treatment of RTH syndromes. In particular, the application of 3,3',5-triiodothyroacetic acid (Triac) in RTH due to defective TRβ and the role of 3,5-diiodothyropropionic acid (DITPA), 3,3',5,5'-tetraiodothyroacetic acid (Tetrac) and Triac in MCT8 deficiency will be highlighted.

Triiodothyroacetic acid in health and disease

Thyroid hormone (TH) is crucial for development and metabolism of many tissues. The physiological relevance and therapeutic potential of TH analogs have gained attention in the field for many years. In particular, the relevance and use of 3,3',5-triiodothyroacetic acid (Triac, TA₃) has been explored over the last decades. Although TA₃ closely resembles the bioactive hormone T₃, differences in transmembrane transport and receptor isoform-specific transcriptional activation potency exist. For these reasons, the application of TA₃ as a treatment for resistance to TH (RTH) syndromes, especially MCT8 deficiency, is topic of ongoing research. This review is a summary of all currently available literature about the formation, metabolism, action and therapeutic applications of TA₃.

Treatment of congenital thyroid dysfunction: Achievements and challenges

The active thyroid hormone tri-iodothyronine (T3) is essential for a normal development of children. Especially within the first years of life, thyroid hormone is pivotal in enabling maturation of complex brain function and somatic growth. The most compelling example for a life without thyroid hormone are those historical cases of children who came to birth without a thyroid gland - as shown in autopsy-studies- and who suffered from untreated hypothyroidism, at that time initially called "sporadic congenital hypothyroidism" (CH). In the last decades huge achievements resulted in a normal development of these children based on newborn screening programs that enable an early onset of a high dose LT4-treatment. Further progress will be necessary to further tailor an individualized thyroid hormone substitution approach and to identify those more complex patients with congenital hypothyroidism and associated defects, who will not benefit from an even optimized LT4 therapy. Besides the primary production of thyroid hormone a variety of further mechanisms are necessary to mediate the function of T3 on normal development that are located downstream of thyroid hormone production. Abnormalities of these mechanisms include the MCT8-transport defect, deiodinase-insufficiency and thyroid hormone receptor alpha-and beta defects. These thyroid hormone resistant diseases can not be treated with classical LT4 substitution alone. The development of new treatment options for those rare cases of thyroid hormone resistance is one of the most challenging tasks in the field of congenital thyroid diseases today.

Guidelines for the treatment of hypothyroidism: prepared by the american thyroid association task force on thyroid hormone replacement

BACKGROUND

A number of recent advances in our understanding of thyroid physiology may shed light on why some patients feel unwell while taking levothyroxine monotherapy. The purpose of this task force was to review the goals of levothyroxine therapy, the optimal prescription of conventional levothyroxine therapy, the sources of dissatisfaction with levothyroxine therapy, the evidence on treatment alternatives, and the relevant knowledge gaps. We wished to determine whether there are sufficient new data generated by well-designed studies to provide reason to pursue such therapies and change the current standard of care. This document is intended to inform clinical decision-making on thyroid hormone replacement therapy; it is not a replacement for individualized clinical judgment.

METHODS

Task force members identified 24 questions relevant to the treatment of hypothyroidism. The clinical literature relating to each question was then reviewed. Clinical reviews were supplemented, when relevant, with related mechanistic and bench research literature reviews, performed by our team of translational scientists. Ethics reviews were provided, when relevant, by a bioethicist. The responses to questions were formatted, when possible, in the form of a formal clinical recommendation statement. When responses were not suitable for a formal clinical recommendation, a summary response statement without a formal clinical recommendation was developed. For clinical recommendations, the supporting evidence was appraised, and the strength of each clinical recommendation was assessed, using the American College of Physicians system. The final document was organized so that each topic is introduced with a question, followed by a formal clinical recommendation. Stakeholder input was received at a national meeting, with some subsequent refinement of the clinical questions addressed in the document. Consensus was achieved for all recommendations by the task force.

RESULTS

We reviewed the following therapeutic categories: (i) levothyroxine therapy, (ii) non-levothyroxine-based thyroid hormone therapies, and (iii) use of thyroid hormone analogs. The second category included thyroid extracts, synthetic combination therapy, triiodothyronine therapy, and compounded thyroid hormones.

CONCLUSIONS

We concluded that levothyroxine should remain the standard of care for treating hypothyroidism. We found no consistently strong evidence for the superiority of alternative preparations (e.g., levothyroxine-liothyronine combination therapy, or thyroid extract therapy, or others) over monotherapy with levothyroxine, in improving health outcomes. Some examples of future research needs include the development of superior biomarkers of euthyroidism to supplement thyrotropin measurements, mechanistic research on serum triiodothyronine levels (including effects of age and disease status, relationship with tissue concentrations, as well as potential therapeutic targeting), and long-term outcome clinical trials testing combination therapy or thyroid extracts (including subgroup effects). Additional research is also needed to develop thyroid hormone analogs with a favorable benefit to risk profile.

Treatment with thyroid hormone

Thyroid hormone deficiency can have important repercussions. Treatment with thyroid hormone in replacement doses is essential in patients with hypothyroidism. In this review, we critically discuss the thyroid hormone formulations that are available and approaches to correct replacement therapy with thyroid hormone in primary and central hypothyroidism in different periods of life such as pregnancy, birth, infancy, childhood, and adolescence as well as in adult patients, the elderly, and in patients with comorbidities. Despite the frequent and long term use of l-T4, several studies have documented frequent under- and overtreatment during replacement therapy in hypothyroid patients. We assess the factors determining l-T4 requirements (sex, age, gender, menstrual status, body weight, and lean body mass), the major causes of failure to achieve optimal serum TSH levels in undertreated patients (poor patient compliance, timing of l-T4 administration, interferences with absorption, gastrointestinal diseases, and drugs), and the adverse consequences of unintentional TSH suppression in overtreated patients. Opinions differ regarding the treatment of mild thyroid hormone deficiency, and we examine the recent evidence favoring treatment of this condition. New data suggesting that combined therapy with T3 and T4 could be indicated in some patients with hypothyroidism are assessed, and the indications for TSH suppression with l-T4 in patients with euthyroid multinodular goiter and in those with differentiated thyroid cancer are reviewed. Lastly, we address the potential use of thyroid hormones or their analogs in obese patients and in severe cardiac diseases, dyslipidemia, and nonthyroidal illnesses.

Management of differentiated thyroid cancer in the presence of resistance to thyroid hormone and TSH-secreting adenomas: a report of four cases and review of the literature

BACKGROUND

An increased or normal serum TSH concentration, despite elevated thyroid hormone levels, is observed in resistance to thyroid hormone (RTH) and TSH-secreting adenomas (TSHomas). When coexistent with a differentiated thyroid cancer (DTC), maintenance of a suppression of TSH is challenging.

OBJECTIVES

The aim of the study was to discuss the pitfalls arising from the failure to suppress TSH secretion in DTC and the strategies for proper treatment of DTC in association with RTH and TSHoma.

METHODS

Four unusual cases of DTC associated with TSHoma (2 cases), RTH (1 case), and an elevated TSH of unknown etiology (1 case) are presented, and the literature is reviewed.

RESULTS

Although a persistent mild TSH elevation may not be a risk factor for the development of DTC, it represents an important problem during the treatment of DTC. Aggressive treatment options should be applied in the proper order to prevent tumor recurrence and persistence in the absence of ideal TSH suppression.

CONCLUSIONS

Although there is no agreed consensus regarding the management of DTC in the presence of persistent hyperthyrotropinemia, complete tumor removal followed by radioablation and attempts to reduce the serum TSH to the lowest tolerable level are recommended. The outcomes in the reported cases have not been unfavorable, despite the persistence of nonsuppressed TSH.

Long-term 3,5,3'-triiodothyroacetic acid therapy in a child with hyperthyroidism caused by thyroid hormone resistance: pharmacological study and therapeutic recommendations

BACKGROUND

The effectiveness of short-term 3,5,3'-triiodothyroacetic acid (TRIAC) therapy for the treatment of hyperthyroidism caused by thyroid hormone resistance (RTH) has been documented. Here, we report a 3-year course of TRIAC therapy in an RTH boy, with a quantitative evaluation of the therapeutic effects and pharmacological study of TRIAC.

PATIENT FINDINGS

The gene encoding the thyroid hormone receptor beta (THRB) of the patient carries a P453T mutation. During treatment with up to 3.0 mg TRIAC per day, reduction in the thyroid volume, resolution of supraventricular arrhythmia, and decrease in thyroid-stimulating hormone (TSH) and free-thyroxine (FT4) levels were achieved. In addition, attention-deficit hyperactivity disorder (ADHD) symptoms improved, with a concomitant decline in the ADHD Rating Scale score.

SUMMARY

A TRIAC pharmacokinetic study, conducted using triiodothyronine level as a surrogate for TRIAC level, demonstrated that TRIAC disappears from the circulation rapidly and has a shorter duration of TSH secretion inhibitory effect in the RTH patient compared to that in the control subject. Studies of TSH and FT4 levels over a period of 3 years indicated that the TRIAC effect is dose dependent.

CONCLUSIONS

TRIAC was effective and safe in ameliorating the effects of hyperthyroidism and ADHD symptoms in a child with known genetic RTH. Further, it was demonstrated that TRIAC has a short half-life and functions dose dependently.

The syndromes of reduced sensitivity to thyroid hormone

BACKGROUND

Six known steps are required for the circulating thyroid hormone (TH) to exert its action on target tissues. For three of these steps, human mutations and distinct phenotypes have been identified.

SCOPE OF REVIEW

The clinical, laboratory, genetic and molecular characteristics of these three defects of TH action are the subject of this review. The first defect, recognized 45years ago, produces resistance to TH and carries the acronym, RTH. In the majority of cases it is caused by TH receptor β gene mutations. It has been found in over 3000 individuals belonging to approximately 1000 families. Two relatively novel syndromes presenting reduced sensitivity to TH involve membrane transport and metabolism of TH. One of them, caused by mutations in the TH cell-membrane transporter MCT8, produces severe psychomotor defects. It has been identified in more than 170 males from 90 families. A defect of the intracellular metabolism of TH in 10 individuals from 8 families is caused by mutations in the SECISBP2 gene required for the synthesis of selenoproteins, including TH deiodinases.

MAJOR CONCLUSIONS

Defects at different steps along the pathway leading to TH action at cellular level can manifest as reduced sensitivity to TH.

GENERAL SIGNIFICANCE

Knowledge of the molecular mechanisms involved in TH action allows the recognition of the phenotypes caused by defects of TH action. Once previously known defects have been ruled out, new molecular defects could be sought, thus opening the avenue for novel insights in thyroid physiology. This article is part of a Special Issue entitled Thyroid hormone signaling.

Variable clinical presentation and outcome in pediatric patients with resistance to thyroid hormone (RTH)

Resistance to thyroid hormone (RTH) is characterized by elevated levels of thyroid hormones, normal or slightly increased TSH levels respondent to TRH, resistance to thyroid hormone administration, and variable clinical expression. To describe the diverse clinical and biochemical findings of six children from five unrelated families with molecular diagnosis of RTH (0.5-12.7 years) and their follow-up (3-20 years). All RTH patients and 4 affected parents' harbored mutations in exons 9 or 10 of the thyroid receptor β gene: p.M313T (de novo), pN331D, p.L341P, p.L346F, and p.P453L. At consultation 5/6 had goiter, 4/6 tachycardia, and 3/5 learning disabilities. Median hormone levels were: T(4) 257.4 nmol/l (NR: 77.2-180.2); FreeT(4) 39.9 pmol/(NR:10.3-28.3); T(3) 4.28 nmol/l (NR:1.23-3.39) TSH 2.8 mUI/l (NR: 0.5-5) always responsive to TRH. TSH levels remained detectable after supraphysiologic T(3) administration while SHBG levels showed a paradoxical decrease in 4/6. Thyroid antibodies, initially present in two subjects, became positive in other two during follow-up. All patients grew normally and presented variable symptoms that were treated according to need. Two patients developed psychiatric disorders. Only one of the four affected parents exhibited clinical signs of RTH (tachycardia and depression). Parent's thyroid profile showed similar TSH and T(3) levels but lower T(4) and FT(4) than their children. RTH has a distinctive biochemical profile with highly variable clinical manifestations and outcomes. Its recognition and molecular characterization avoid misleading diagnosis. Treatment has to be instituted according to each subject's own clinical requirements.

Pituitary resistance to thyroid hormones: pathophysiology and therapeutic options

Thyroid hormone secretion suppresses the expression of thyroid stimulating hormone (TSH), both of which are strictly controlled by a negative feedback loop between the hypothalamus-pituitary and thyroid. Pituitary resistance to thyroid hormone (PRTH) is defined as resistance to the action of thyroid hormone that is more severe in the pituitary than at the peripheral tissue level. Although the molecular basis of PRTH is not well understood, the clinical issue mainly involves imbalance between the hypothalamus-pituitary and peripheral thyroid hormone responsivity, which may induce peripheral thyrotoxic phenomena. Here, we review the pathogenesis and molecular aspects of PRTH, present a single case with inappropriate TSH secretion suffering from thyrotoxicosis treated with PTU, and discuss the possible choice of therapeutic options to correct the imbalance of thyroid hormone responsivity in both the hypothalamus-pituitary and peripheral tissues.

Approach to the patient with resistance to thyroid hormone and pregnancy

Resistance to thyroid hormone (RTH), a syndrome of reduced end-organ responsiveness to thyroid hormone (TH), is mostly caused by mutations in the TH receptor (TR) beta gene. Diagnosis is based on persistent elevations of serum free T(4) and often T(3) levels in the absence of TSH suppression, and confirmation in most cases is by way of genetic testing. The mainstay in the management of RTH patients who are asymptomatic is to recognize the correct diagnosis and avoid antithyroid treatment. Deciding whether to manage these patients with TH replacement is made even more challenging when an affected individual is pregnant. How one approaches such a patient with pregnancy and RTH would depend on the genotype of the fetus. This requires obtaining prenatal information on the genotype of the fetus and a thorough history of the outcome of previous pregnancies as well as a history of the course and outcome of other family members with RTH. If the TRbeta mutation is known in the mother, the fetus can be rapidly genotyped from DNA from amniocentesis for the same mutation, and then management decisions could be made regarding thyroid or antithyroid hormone treatment.

[Therapeutic possibilities in patients with selective pituitary resistance to thyroid hormones]

Selective pituitary resistance to thyroid hormones (SPRTH) is a non-neoplastic form of inappropriate secretion of thyrotropin (TSH). The etiology of this hormonal resistance is linked to inactivating mutations in the thyroid hormone receptor beta (TR-beta) gene. These mutations affect critical portions of the receptor's triiodothyronine (T3)-binding domain. Clinically, SPRTH is characterized by hyperthyroidism with goiter and absence of pituitary mass in the morphologic study. Laboratory data show an elevation of free T3 and free thyroxine concentrations without suppression of TSH, with normal molar subunit alpha/TSH ratio. At this time, there is no specific therapy for SPRHT. Beta blockers, such as atenolol, and benzodiazepines have been used as a symptomatic therapy. Among the drugs with the capacity for reducing TSH secretion are TR agonists, such as triiodothyroacetic acid, D-thyroxine, triiodothyropropionic acid, and L-T3.

Thermogenic effect of triiodothyroacetic acid at low doses in rat adipose tissue without adverse side effects in the thyroid axis

Triiodothyroacetic acid (TRIAC) is a physiological product of triiodothyronine (T(3)) metabolism, with high affinity for T(3) nuclear receptors. Its interest stems from its potential thermogenic effects. Thus this work aimed 1) to clarify these thermogenic effects mediated by TRIAC vs. T(3) in vivo and 2) to determine whether they occurred predominantly in adipose tissues. To examine this, control rats were infused with equimolar T(3) or TRIAC doses (0.8 or 4 nmolx100 g body wt(-1) x day(-1)) or exposed for 48 h to cold. Both T(3) doses and only the highest TRIAC dose inhibited plasma and pituitary thyroid-stimulating hormone (TSH) and thyroxine (T(4)) in plasma and tissues. Interestingly, the lower TRIAC dose marginally inhibited plasma T(4). T(3) infusion increased plasma and tissue T(3) in a tissue-specific manner. The highest TRIAC dose increased TRIAC concentrations in plasma and tissues, decreasing plasma T(3). TRIAC concentrations in tissues were <10% those of T(3). Under cold exposure or high T(3) doses, TRIAC increased only in white adipose tissue (WAT). Remarkably, only the lower TRIAC dose activated thermogenesis, inducing ectopic uncoupling protein (UCP)-1 expression in WAT and maximal increases in UCP-1, UCP-2, and lipoprotein lipase (LPL) expression in brown adipose tissue (BAT), inhibiting UCP-2 in muscle and LPL in WAT. TRIAC, T(3), and cold exposure inhibited leptin secretion and mRNA in WAT. In summary, TRIAC, at low doses, induces thermogenic effects in adipose tissues without concomitant inhibition of TSH or hypothyroxinemia, suggesting a specific role regulating energy balance. This selective effect of TRIAC in adipose tissues might be considered a potential tool to increase energy metabolism.

Syndromes of hormone resistance in the hypothalamic-pituitary-thyroid axis

Forty years have elapsed since the first description of a syndrome of resistance in the hypothalamic-pituitary-thyroid axis, i.e., resistance to thyroid hormone action. In the last two decades many other types of resistance have been discovered, including resistance to the action of thyrotropin-releasing hormone (TRH), of thyroid-stimulating hormone (TSH), and of thyroid hormones (THs); the latter can be due not only to thyroid hormone receptor defects but also to alteration in genes encoding TH-specific transporters or components involved in metabolic pathways of THs. Moreover, alteration in genes encoding for second messengers may cause forms of resistance other than those due to receptor mutations, the most important one being that of an inactivating mutation in the G-protein alpha-subunit leading to TSH resistance in the setting of pseudohypoparathyroidism type 1a. Recognition of these rare thyroid disorders is of great importance not only for informed genetic counselling but also for avoiding diagnostic mistakes that may lead to incorrect and potentially dangerous treatments.

Alternate pathways of thyroid hormone metabolism

The major thyroid hormone (TH) secreted by the thyroid gland is thyroxine (T(4)). Triiodothyronine (T(3)), formed chiefly by deiodination of T(4), is the active hormone at the nuclear receptor, and it is generally accepted that deiodination is the major pathway regulating T(3) bioavailability in mammalian tissues. The alternate pathways, sulfation and glucuronidation of the phenolic hydroxyl group of iodothyronines, the oxidative deamination and decarboxylation of the alanine side chain to form iodothyroacetic acids, and ether link cleavage provide additional mechanisms for regulating the supply of active hormone. Sulfation may play a general role in regulation of iodothyronine metabolism, since sulfation of T(4) and T(3) markedly accelerates deiodination to the inactive metabolites, reverse triiodothyronine (rT(3)) and T(2). Sulfoconjugation is prominent during intrauterine development, particularly in the precocial species in the last trimester including humans and sheep, where it may serve both to regulate the supply of T(3), via sulfation followed by deiodination, and to facilitate maternal-fetal exchange of sulfated iodothyronines (e.g., 3,3'-diiodothyronine sulfate [T(2)S]). The resulting low serum T(3) may be important for normal fetal development in the late gestation. The possibility that T(2)S or its derivative, transferred from the fetus and appearing in maternal serum or urine, can serve as a marker of fetal thyroid function is being studied. Glucuronidation of TH often precedes biliary-fecal excretion of hormone. In rats, stimulation of glucuronidation by various drugs and toxins may lead to lower T(4) and T(3) levels, provocation of thyrotropin (TSH) secretion, and goiter. In man, drug induced stimulation of glucuronidation is limited to T(4), and does not usually compromise normal thyroid function. However, in hypothyroid subjects, higher doses of TH may be required to maintain euthyroidism when these drugs are given. In addition, glucuronidates and sulfated iodothyronines can be hydrolyzed to their precursors in gastrointestinal tract and various tissues. Thus, these conjugates can serve as a reservoir for biologically active iodothyronines (e.g., T(4), T(3), or T(2)). The acetic acid derivatives of T(4), tetrac and triac, are minor products in normal thyroid physiology. However, triac has a different pattern of receptor affinity than T(3), binding preferentially to the beta receptor. This makes it useful in the treatment of the syndrome of resistance to thyroid hormone action, where the typical mutation affects only the beta receptor. Thus, adequate binding to certain mutated beta receptors can be achieved without excessive stimulation of alpha receptors, which predominate in the heart. Ether link cleavage of TH is also a minor pathway in normal subjects. However, this pathway may become important during infections, when augmented TH breakdown by ether-link cleavage (ELC) may assist in bactericidal activity. There is a recent claim that decarboxylated derivates of thyronines, that is, monoiodothyronamine (T(1)am) and thyronamine (T(0)am), may be biologically important and have actions different from those of TH. Further information on these interesting derivatives is awaited.

Syndromes of thyroid hormone resistance

Thyroid hormone resistance (RTH) is a rare autosomal dominant disorder, characterized clinically by goiter and biochemically by elevated circulating free thyroid hormone levels in the presence of measurable serum TSH concentrations. About 85% of patients with RTH are harboring mutations in thyroid hormone receptor beta (TRB). These mutations cluster in three different "hot spot" in the T3 binding domain of the receptor. When mapped to their homologous residues in the TR crystal structure, these three clusters of mutations border the T3-binding pocket. As a consequence, most TRB mutations impair the hormone binding to the receptor and interfere with the mechanism(s) of corepressor release and the consequent recruitment of coactivators. Thus, the remodeling of chromatin structure throughout the process of histone acetylation is prevented and the transcriptional activity of the mutant receptor on both positively and negatively regulated genes, severely disrupted. The lack of interaction with coactivators appears to be an additional mechanism for the dominant negative effects of mutant TRB on the transcriptional activity of the normal receptor.

In vitro effect of Triac on resistance to thyroid hormone receptor mutants: potential basis for therapy

Resistance to thyroid hormone (RTH) is a syndrome caused by a mutation in the carboxyl-terminal domain of the thyroid hormone receptor beta (TRbeta) gene. 3,5,3'-triiodothyroacetic acid (Triac) has been used on an empirical basis to treat RTH but its efficacy is still controversial. In previous studies, we demonstrated that Triac has TR isoform- and TRE-specific effects. In this report, we used five natural RTH mutations of the ligand-binding domain in both TRbeta1 and TRbeta2 isoforms for the evaluation of the effect of T3 and Triac on regulation of transcription and binding affinity. We show that Triac has superior activity on negatively and positively regulated promoters and higher binding affinity than T3 for a majority of TRbeta1 and TRbeta2 mutants. However, the difference of transcriptional activity and binding affinity between both ligands is less for RTH mutants than for wild type receptors. These results suggest that Triac could be a potential treatment for RTH patients.

A novel mutation (M310L) in the thyroid hormone receptor beta causing resistance to thyroid hormone in a Brazilian kindred and a neonate

Resistance to thyroid hormone (RTH) is an inherited syndrome of reduced tissue responsiveness to thyroid hormone (T3) caused by mutations in the thyroid hormone receptor beta (TRbeta). The index patient of the family reported here, a 17-year-old woman, came to medical attention because of a diffuse goiter, short stature, and learning disabilities. Biochemical tests revealed an elevated free T4 of 5.2 ng/dl (0.8-2.1), a T3 of 270 ng/dl (80-220), and a nonsuppressed TSH of 1.79 mU/l (0.4-4). Administration of exogenous T4 or T3 did not result in the usual TSH suppression, prompting the clinical diagnosis of RTH. Her father and one of her brothers also had clinical and biochemical findings consistent with RTH. Direct sequence analysis of the TRbeta gene revealed a heterozygous transition 928A>G in exon 9 resulting in substitution of methionine 310 by leucine (M310L). This novel receptor mutant has a reduced affinity for T3 ( approximately 10% of normal) and dominant negative properties that are similar in comparison to other RTH mutations. The index patient had a normal pregnancy and delivery. At birth, the female neonate had no goiter, a significantly elevated T4, and increased TSH. The diagnosis of RTH was confirmed by sequencing the TRbeta gene. She was underweight at birth and her length was between the 5th and 10th percentile. At 26 months, her height remained at the 10th percentile but her bone age was 18 months, suggesting mild hypothyroidism at the level of the bone. In contrast, increased heart rate and restlessness are consistent with hyperthyroidism in other tissues, such as the heart and possibly the brain.

Generalized resistance to thyroid hormone associated with possible selective cardiac nonresistance

OBJECTIVE

To review the condition of generalized resistance to thyroid hormone and to report a case of generalized thyroid hormone resistance associated with atrial fibrillation.

METHODS

A case report is presented of a 52-year-old man with atrial fibrillation who was referred by a cardiologist for thyroid ablation because of "hyperthyroidism," when his free thyroxine was found to be 4.35 ng/dL (normal, 0.55 to 2.46) and his free triiodothyronine was 6.5 pg/mL (normal, 1.4 to 4.4).

RESULTS

This clinically euthyroid man with no signs or symptoms of hyperthyroidism except for the possibly related atrial fibrillation had a thyrotropin level of 3.45 mIU/L (normal, 0.46 to 4.7) in conjunction with the aforementioned increased levels of thyroid hormones. Further evaluation revealed normal 6-hour (11.7%) and 24-hour (27.6%) (123)I uptakes. Magnetic resonance imaging of the pituitary revealed a normal-sized gland with no masses.

CONCLUSION

This is a rare case of generalized resistance to thyroid hormone in a patient with only atrial fibrillation. Whether the heart was selectively nonresistant to thyroid hormone as the cause of his atrial fibrillation or whether his atrial fibrillation was due to his mitral valve prolapse documented on echocardiography could not be determined with certainty. His ventricular rate of 83 per minute and laboratory evaluation suggest that thyroid hormone was not the cause of the atrial fibrillation.

An unusual case of inappropriate secretion of thyrotropin: neoplastic or nonneoplastic?

OBJECTIVE

To report an unusual case that illustrates the difficulties in distinguishing neoplastic (nIST) from nonneoplastic inappropriate secretion of thyrotropin (nnIST).

METHODS

We describe clinical, biochemical, genetic analysis, and magnetic resonance imaging (MRI) results in a patient with hyperthyroidism due to IST, and we review the related literature.

RESULTS

The patient demonstrated overt clinical and biochemical hyperthyroidism with inappropriately increased thyrotropin levels, which failed to respond to thyrotropin-releasing hormone (TRH) stimulation. Sex hormone-binding globulin (SHBG) levels were in the hyperthyroid range. Alpha subunit levels were normal, as was the alpha subunit/thyrotropin molar ratio. MRI of the pituitary was negative for tumor during a 2-year period, and octreotide scan was also negative for sellar uptake. Basal oxygen consumption was abnormally increased. Genetic analysis failed to reveal mutations of the thyroid receptor b gene. The patient responded well to radioiodine ablation of his thyroid.

CONCLUSION

This patient had clinical symptoms of hyperthyroidism associated with some features characteristic of nIST (increased level of SHBG, lack of thyrotropin response to TRH stimulation, absence of thyroid receptor b mutations) and others typical of nnIST (normal alpha subunit and its molar ratio to thyrotropin, absence of tumor on sellar imaging). Close follow-up with periodic MRI of the sella is important because of the possible existence of a small pituitary tumor, which may become apparent at a later date. Therapy to control symptoms is important. Hormone replacement, if needed, should be adjusted to maintain clinical euthyroidism, guided by free thyroxine levels.

Prenatal diagnosis of thyroid hormone resistance

A 29-yr-old woman with pituitary resistance to thyroid hormones (PRTH) was found to harbor a novel point mutation (T337A) on exon 9 of the thyroid hormone receptor beta (TRbeta) gene. She presented with symptoms and signs of hyperthyroidism and was successfully treated with 3,5,3'-triiodothyroacetic acid (TRIAC) until the onset of pregnancy. This therapy was then discontinued in order to prevent TRIAC, a compound that crosses the placental barrier, from exerting adverse effects on normal fetal development. However, as the patient showed a recurrence of thyrotoxic features after TRIAC withdrawal, we sought to verify, by means of genetic analysis and hormone measurements, whether the fetus was also affected by RTH, in order to rapidly reinstitute TRIAC therapy, which could potentially be beneficial to both the mother and fetus. At 17 weeks gestation, fetal DNA was extracted from chorionic villi and was used as a template for PCR and restriction analysis together with direct sequencing of the TRbeta gene. The results indicated that the fetus was also heterozygous for the T337A mutation. Accordingly, TRIAC treatment at a dose of 2.1 mg/day was restarted at 20 weeks gestation. The mother rapidly became euthyroid, and the fetus grew normally up to 24 weeks gestation. At 29 weeks gestation mild growth retardation and fetal goiter were observed, prompting cordocentesis. Circulating fetal TSH was very high (287 mU/L) with a markedly reduced TSH bioactivity (B/I: 1.1 +/- 0.4 vs 12.7 +/- 1.2), while fetal FT4 concentrations were normal (8.7 pmol/L; normal values in age-matched fetuses: 5-22 pmol/L). Fetal FT3 levels were raised (7.1 pmol/L; normal values in age-matched fetuses: <4 pmol/L), as a consequence of 100% cross-reactivity of TRIAC in the FT3 assay method. To reduce the extremely high circulating TSH levels and fetal goiter, the dose of TRIAC was increased to 3.5 mg/day. To monitor the possible intrauterine hypothyroidism, another cordocentesis was performed at 33 weeks gestation, showing that TSH levels were reduced by 50% (from 287 to 144 mU/L). Furthermore, a simultaneous ultrasound examination revealed a clear reduction in fetal goiter. After this latter cordocentesis, acute complications occured, prompting delivery by cesarean section. The female neonate was critically ill, with multiple-organ failure and respiratory distress syndrome. In addition, a small goiter and biochemical features ofhypothyroidism were noted transiently and probably related to the prematurity of the infant. At present, the baby is clinically euthyroid, without goiter, and only exhibits biochemical features of RTH. In summary, although further fetal studies in cases of RTH are necessary to determine whether elevated TSH levels with a markedly reduced bioactivity are a common finding, our data suggest transient biochemical hypothyroidism in RTH during fetal development. Furthermore, we advocate prenatal diagnosis of RTH and adequate treatment of the disease in case of maternal hyperthyroidism, to avoid fetal thyrotrope hyperplasia, reduce fetal goiter, and maintain maternal euthyroidism during pregnancy.

Thyroid disease mediated by molecular defects in cell surface and nuclear receptors

The proposed mechanisms of RTH are not mutually exclusive. In fact, there is considerable experimental evidence that many if not all of these complex receptor interactions with elements of the transcriptional unit are involved in RTH. Several aspects of RTH remain unclear, in particular on a clinical level. We still do not completely understand the seeming paradox of a tight distribution of receptor mutations and wide variability in phenotypic presentation. The discovery that many of the RTH receptors have defects in corepressor interaction makes it tempting to speculate that the variability in RTH phenotype within kindreds is secondary to differences in corepressor expression. These issues may be better understood as research further proceeds into cofactors and their control of transcription. We also need better tools to determine thyroid status at a peripheral level. Basal metabolic rate, serum measurement of thyroid-responsive gene products, echocardiographic techniques, and other clinical measures have for the most part been unhelpful in determining thyroid status of specific organ systems. Consequently, therapeutic interventions for RTH are directed toward normalizing biochemical indices of thyroid homeostasis, without really knowing whether these efforts correct imbalances within crucial tissues. These studies, and the more widespread investigation of hormone receptor action in general, are moving at a breathtaking pace, and there is a keen interest in applying these principals to understanding the pathophysiologic mechanism of a variety of diseases.

Central hyperthyroidism

Central hyperthyroidism is a rare condition in which thyrotoxicosis results from primary overproduction of TSH by the pituitary gland with subsequent thyroid enlargement and hyperfunction. The two known causes of central hyperthyroidism are TSH-producing pituitary tumors (TSHomas) and the syndrome of PRTH. Both of these entities are characterized by clinical thyrotoxicosis, diffuse goiters, elevated circulating levels of free T4 and T3, and a nonsuppressed serum TSH. It is critical to distinguish central hyperthyroidism from the much more common types of primary hyperthyroidism, all of which have undetectable TSH values. TSHomas and PRTH can usually be differentiated from one another by measuring the serum alpha-subunit and the TSH response to intravenous TRH or exogenous thyroid hormone, and by pituitary imaging studies. TSHomas are usually benign adenomas arising from the monoclonal expansion of neoplastic thyrotropes. Causative oncogenes have not yet been convincingly identified. PRTH is a nonneoplastic disorder caused by inherited mutations in the gene for the thyroid hormone receptor beta; it is a poorly understood variant of GRTH. For unclear reasons, in PRTH, the pituitary gland is resistant to the feedback inhibitory effects of circulating thyroid hormones while peripheral tissues respond normally, causing patients to experience the toxic peripheral effects of thyroid hormone excess. TSHomas are best treated by transphenoidal surgical removal. Radiotherapy is indicated for inoperable or incompletely resected tumors. Octreotide administration is a useful adjunct for preoperatively reducing tumor size and for the medical management of surgical treatment failures. PRTH is ideally treated by chronically suppressing TSH secretion with medications such as D-thyroxine, TRIAC, octreotide, or bromocriptine. If such therapy is ineffective or unavailable, thyroid ablation with radioiodine or surgery may be employed with subsequent close monitoring of both thyroid hormone status and pituitary gland size.

Clinical and hormonal outcome after two years of triiodothyroacetic acid treatment in a child with thyroid hormone resistance

We report here a new family with thyroid hormone resistance (RTH), with phenotypic variability among subjects. Particular emphasis is given to the clinical and hormonal outcome after 2 years of triiodothyroacetic acid (TRIAC) treatment in an affected child with peripheral thyrotoxic features (pituitary RTH [PRTH]). The genetic defect was a substitution in position 1642 (C to A) within the exon 10 of thyroid hormone receptor beta1 (TRbeta1) gene, resulting in the codon change P453T. The mutant receptor had a significantly reduced triiodothyronine (T3) binding affinity. Within this family, the child and the mother suffered from hyperthyroidism and were clinically classified as PRTH, while the maternal grandmother was clinically euthyroid, indicating a generalized form of the disease (GRTH). Rapid normalization of heart rate was initially obtained by the association of the cardioselective beta-blocker atenolol with TRIAC. Nevertheless, long-term TRIAC therapy, through its lowering action of serum thyrotropin (TSH) and thyroid hormone levels, maintained a normal heart rate after atenolol discontinuation and normalized the neurological disturbances and the clinical signs in the child, without any apparent side effect. In fact, growth velocity remained unchanged and no alteration of several parameters of thyroid hormone action at the tissue level was observed, whereas soluble interleukin-2 receptor levels improved significantly, confirming the safety and efficacy of long-term TRIAC therapy for PRTH also during childhood. We thus recommend testing the efficacy of TRIAC therapy in all RTH patients presenting with clinical features of hyperthyroidism.

Hormone-nuclear receptor interactions in health and disease. Thyroid hormone resistance

The syndromes of resistance to thyroid hormone (RTH) are rare disorders characterized by elevated levels of circulating free thyroid hormones, inappropriate TSH secretion and variably reduced peripheral tissue responses to iodothyronine action. On the basis of clinical features, two major forms of RTH are recognized: generalized resistance (GRTH) in which patients are asymptomatic with few clinical signs, and pituitary resistance (PRTH) where patients present with features associated with thyrotoxicosis. However, a review of the literature and our own experience indicates that there is a wide overlap of clinical and biochemical features between individuals with GRTH or PRTH. Genetic analysis shows that both disorders are associated with a number of different mutations in the thyroid hormone receptor beta (TR-beta) gene which localize to two regions in the hormone-binding domain. The mutant proteins are transcriptionally impaired but preserve the ability to bind DNA, dimerize and inhibit the function of their wild-type counterparts in a dominant negative manner. Dominant negative effects of mutant receptors within the pituitary-thyroid feedback axis generate abnormal thyroid function test results characteristic of RTH. The variable peripheral resistance may be related to differences in tissue distribution of TR-alpha versus TR-beta receptor isoforms, variable dominant negative effects of mutant receptors on different target genes or other factors not related to the receptor mutation. Although GRTH and PRTH represent the variable phenotypic spectrum of a single genetic entity, this clinical distinction will remain useful as a guide to appropriate treatment.

The variable clinical phenotype in thyroid hormone resistance syndrome

Thyroid hormone resistance syndrome (RTH) is a rare disorder characterized by elevated levels of circulating free thyroid hormones, inappropriate TSH secretion, and reduced peripheral tissue responses to iodothyronine action. On the basis of clinical features, at least two different forms of RTH have been described: generalized resistance (GRTH) in which patients are asymptomatic with few clinical signs and pituitary resistance (PRTH) where patients present with some signs and symptoms associated with thyrotoxicosis. However, a review of the literature and our own experience indicates that there is a wide overlap of symptoms and signs exhibited by individuals with GRTH or PRTH. Assessments using biochemical and physiological indices of thyroid hormone action are useful, but limited by their lack of precision and also show an overlap between values recorded in GRTH and PRTH. In addition, we have observed significant temporal variations in clinical signs as well as in parameters of thyroid hormone action in the same individuals, with no correlation with their subjective symptoms. Recent genetic analyses indicate that patients with either GRTH or PRTH are heterozygous for mutations in the thyroid hormone receptor beta (TR beta) gene. Indeed, different clinical features have been observed in affected individuals within a kindred harboring the same Tr beta mutation, and identical mutations have been identified in unrelated kindreds classified as GRTH or PRTH. These data support the view that GRTH and PRTH are variable manifestations of a single genetic entity. Nevertheless, this clinical distinction will remain useful as a guide to the most appropriate treatment. The variable phenotypic spectrum of thyroid hormone resistance may be related to factors other than mutations in Tr beta that have yet to be elucidated.

Rabbit skeletal muscle sarcoplasmic reticulum Ca(2+)-ATPase activity: stimulation in vitro by thyroid hormone analogues and bipyridines

Sarcoplasmic reticulum-enriched membranes from rabbit skeletal muscle contained Ca(2+)-ATPase activity which was significantly enhanced (26% increase, P < 0.001) in vitro by physiological concentrations (10(-10) M) of L-thyroxine (T4) and 3,3',5-triiodo-L-thyronine (T3). In contrast, the biologically inactive iodothyronine analogues D-T4 and 3,3',5,5'-tetraiodothyroacetic acid (Tetrac) (10(-10) M) were without effect on enzyme activity. 3,5-Dimethyl-3'-isopropyl-L-thyronine (Dimit), a bioactive analogue, was highly effective as a Ca(2+)-ATPase stimulator, increasing enzyme activity by 43% (P < 0.02 vs. T4 effect). A bipyridine cardiac inotropic agent, milrinone, has been reported to be thyromimetic in a myocardial membrane Ca(2+)-ATPase system, and in concentrations from 10(-10) to 10(-5) M enhanced skeletal muscle SR membrane Ca(2+)-ATPase activity in vitro (P < 0.001). Milrinone analogues which have been previously shown to enhance rabbit myocardial membrane Ca(2+)-ATPase activity, and which have a twist relationship of the pyridine rings, were also striated muscle Ca(2+)-ATPase stimulators. We conclude that (1) striated muscle is a mammalian tissue in which physiological levels of biologically relevant thyroid hormone analogues, particularly Dimit, stimulate Ca(2+)-ATPase activity in vitro by a non-genomic mechanism; (2) cardiac bipyridine analogues which are thyromimetic in vitro in rabbit heart, and which have structural homologies with thyroid hormone, are stimulators of rabbit striated muscle sarcoplasmic reticulum Ca(2+)-ATPase activity.

Thyroid hormone resistance syndromes

The thyroid hormone resistance syndromes are disorders in which the body's tissues are resistant to the effects of thyroid hormone. Generalized resistance to thyroid hormone (GRTH) is characterized by resistance in the pituitary gland and in most or all of the peripheral tissues. Affected individuals have elevated serum thyroid hormone levels and inappropriately normal or elevated thyroid-stimulating hormone (TSH) but are usually clinically euthyroid and require no treatment. Selective pituitary resistance to thyroid hormone (PRTH) is characterized by resistance in the pituitary gland but not in peripheral tissues. Patients have elevated serum thyroid hormone levels and normal or elevated TSH levels and are clinically thyrotoxic. Therapy is usually necessary, but current choices are not completely satisfactory. Selective peripheral resistance to thyroid hormone (PerRTH) is characterized by resistance in peripheral tissues but not in the pituitary. The only patient thus far described had normal serum thyroid hormone and TSH levels but was clinically hypothyroid and improved with thyroid hormone administration. All of these disorders are probably more common than is generally recognized and are often misdiagnosed and inappropriately treated. GRTH, in most cases studied, results from a mutation in the thyroid hormone receptor beta gene causing an amino acid substitution in or a partial or complete deletion of the thyroid hormone-binding domain of the receptor. The causes of PRTH and PerRTH remain to be determined.

Identification of a novel mutation in the gene encoding the beta-triiodothyronine receptor in a patient with apparent selective pituitary resistance to thyroid hormone

OBJECTIVE

We investigated whether the first patient (L-F3) reported as having selective pituitary resistance had a mutation in the hTR beta gene. We compared the clinical parameters of this case with those of patients with generalized resistance to thyroid hormone.

DESIGN

The patient, L-F3, was part of a study at the NIH to identify mutations by sequencing the hTR beta gene in kindreds with thyroid hormone resistance. The clinical data of L-F3 as well as patients with generalized resistance to thyroid hormone were compared and analysed retrospectively.

MEASUREMENT

We amplified by the polymerase chain reaction and then sequenced exons 5 to 10 of the hTR beta gene in L-F3 and a normal control. Upon finding the mutation in L-F3, we measured the affinity constant of this mutated hTR beta receptor. Criteria developed previously were used to assess tissue responsiveness to thyroid hormone of L-F3.

RESULTS

We identified a C to T transition at base 1297 in codon 333 of the hTR beta gene in the first patient (L-F3) reported as having apparent selective pituitary resistance. This base substitution resulted in more than a four-fold decrease in T3-binding affinity for the hTR beta 1 receptor. The mutation of L-F3 occurred in the dimerization domain of exon 9, a region where the majority of mutations of kindreds with generalized thyroid hormone resistance have been found. Furthermore, the nucleotide substitution at base 1297 found in the apparent selective pituitary resistant case, L-F3, was the same as in an unrelated patient (K-T3) with generalized resistance to thyroid hormone. As a result, we compared the clinical parameters of both patients and found that they had similar patterns of resistance in several tissues. Besides the bone resistance present in both kindreds, the apparent selective pituitary resistance case also had liver and neuromuscular resistance.

CONCLUSIONS

These findings suggest that apparent selective pituitary resistance and generalized resistance to thyroid hormone are not qualitatively different syndromes. Nevertheless, identification of selective pituitary resistance is a useful clinical distinction since such patients with clinical and biochemical features of hyperthyroidism appear to benefit from reduction in serum thyroid hormone concentrations. In contrast, patients with more conventional forms of thyroid hormone resistance require no treatment or may benefit from increased concentrations of thyroid hormone.

Possible etiology for euthyroid sick syndrome

Euthyroid sick syndrome is possibly due to an increase in Triac (a T3 analogue) and/or Tetrac (a T4 analogue). Triac and/or Tetrac possibly feed back on the pituitary/hypothalamus region and cause a secondary hypothyroidism. This is possibly analogous to such diverse conditions as fetal thyroid function and black bear hibernation.

Pituitary resistance to thyroid hormone action with preserved circadian rhythm of thyrotropin in a postmenopausal woman

We assessed the 24-h behavior of circulating TSH and the dopaminergic control on TSH release in a postmenopausal woman, who had elevated levels of serum thyroid hormones and an inappropriately high concentration of serum TSH, indicating pituitary resistance to thyroid hormone action. The patient was found to have a daily profile of serum TSH similar to that of normal subjects, except for the persistently elevated 24 h levels, suggesting that alterations in thyroid hormone negative feedback control did not affect substantially circadian TSH rhythm. The acute administration of a dopamine antagonist drug (metoclopramide) resulted in a markedly elevated peak of serum TSH, similar both in the morning and in the evening. The chronic administration of a dopamine agonist drug (bromocriptine) reduced basal and TRH-stimulated TSH, restored circadian TSH variations to lower levels, and normalized other thyroid function tests. Although the metoclopramide test results confirmed the existence of an increased dopaminergic inhibitory tone in nonneoplastic inappropriate secretion of TSH, the outcome of bromocriptine treatment indicated that the dopaminergic control over TSH release was not enough in this case of PRTH.

Hyperthyroidism due to inappropriate secretion of thyrotropin in 10 patients

PURPOSE

The syndrome of inappropriate thyroid-stimulating hormone (TSH) secretion, characterized by elevated serum free thyroxine and triiodothyronine levels in association with measurable serum TSH concentrations, remains an uncommon cause of hyperthyroidism that is being recognized with increasing frequency. The hyperthyroidism may be due to either neoplastic pituitary TSH secretion or selective pituitary resistance to thyroid hormone. In an effort to better understand this rare cause of hyperthyroidism, we undertook a retrospective analysis of our institution's experience with this condition.

PATIENTS

We reviewed our cumulative experience (10 patients) with hyperthyroidism due to the syndrome of inappropriate secretion of TSH.

RESULTS

Six patients were diagnosed with TSH-secreting pituitary adenomas and four were found to have selective pituitary resistance to thyroid hormone. One patient with tumor had a TSH-secreting pituitary adenoma in the setting of multiple endocrine neoplasia syndrome. In all patients with tumor, hyperthyroidism was successfully treated with transsphenoidal adenomectomy with or without pituitary radiotherapy. All four patients with pituitary resistance had thyroid ablation or resection prior to their correct diagnosis. Therefore, therapy for this group of patients involved thyroid hormone replacement and efforts to suppress TSH hypersecretion. All 10 patients have done well clinically, with follow-up ranging from 2 weeks to 13 years.

CONCLUSIONS

Adequate treatment exists for the two primary causes of TSH hypersecretion. TSH-secreting pituitary adenomas are treated with surgery and, if necessary, adjuvant pituitary radiotherapy. The results are generally good if the tumor is diagnosed and treated at an early stage. Primary therapy for hyperthyroidism due to selective pituitary resistance to thyroid hormone is aimed at suppression of pituitary TSH hypersecretion. The evaluation of any patient with hyperthyroidism must be thorough and, in some cases, should include measurement of TSH to determine the presence of inappropriate secretion. Eliminating this diagnosis will help avoid improper and potentially harmful treatment of hyperthyroid patients.

Dextrothyroxine in the treatment of generalized thyroid hormone resistance in a boy homozygous for a defect in the T3 receptor

The dextroisomer of thyroxine (D-T4) has been shown to have suppressive effects on pituitary TSH secretion in euthyroid individuals and patients with mild thyroid hormone resistance. We treated a 3-year-old boy with D-T4 who was homozygous for a T3 receptor defect, resulting in a complex clinical picture of tissue-specific hyperthyroidism and hypothyroidism. There was no evidence of significant alteration in thyroid physiology, including serum concentrations of basal and TRH stimulated TSH or echocardiographic parameters measuring systolic time interval. We conclude that D-T4 at a daily dose of 6 mg (0.65 mg/kg) was ineffective in this boy with homozygous dominant negative thyroid hormone resistance.

Hyperthyroidism due to familial pituitary resistance to thyroid hormone: successful control with 3, 5, 3' triiodothyroacetic associated to propranolol

We herein describe a family with thyroid hormone resistance. Thyroid hormones and basal TSH were elevated. Pituitary tumor or abnormality in thyroid hormone binding proteins were ruled out by appropriate tests. Mother and sister of the propositus presented similar abnormal hormonal features but no hyperthyroidism. Initially the patient was treated with carbimazole (30 mg/day): three months later a dramatic increase in the size of the thyroid gland and in TSH levels (12.5 to 28 mU/l) were noted. Thereafter, dextrothyroxine (D-T4) and 3, 5, 3'-triiodothyroacetic acid (TRIAC) were given consecutively and treatment was accompanied by a decrease of TSH levels (2 mU/l) but thyroid hormone remained elevated. The symptoms and signs of hyperthyroidism improved with the addition of propranolol (30-60 mg/day). In conclusion, the present report describes a new family with the syndrome of THR and variable degrees of involvement among relatives. We suggest the usefulness of TRIAC therapy to decrease TSH levels and propranolol to improve thyrotoxicosis due to pituitary resistance to thyroid hormone.

Clinical hyperthyroidism due to non-neoplastic inappropriate thyrotrophin secretion

We report a case of hyperthyroidism due to inappropriate thyrotrophin (TSH) secretion in a patient with selective pituitary resistance to thyroid hormone action. Symptoms of hyperthyroidism in patients with this disorder are usually mild, implying some peripheral tissue resistance to the metabolic effects of thyroid hormone. Our patient had unusually severe symptoms, including marked weight loss and cardiac arrythmias which required carbimazole and beta-blocker therapy for control. Somatostatin was ineffective in suppressing TSH secretion. The introduction of sensitive thyrotrophin assays should facilitate the accurate diagnosis of TSH-induced hyperthyroidism and avoid inappropriate treatment.

Thyrotoxicosis due to pituitary resistance to thyroid hormones. Successful control with D thyroxine: a study in three patients

Selective pituitary resistance to thyroid hormone (PRTH) is responsible for thyrotoxicosis due to inappropriate secretion of TSH. The TSH suppressive action of D-thyroxine (DT4) has been previously documented in euthyroid and hypothyroid subjects. This prompted us to treat with DT4 three patients with PRTH uncontrolled by anti-thyroid drugs (ATD) alone or supplemented with bromocriptine, and whose follow-up had been complicated by atrial fibrillation in two patients. Because of 100% cross-reactivity between the D and L isomers of T4 and T3 in our RIAs, thyroglobulin (Tg) was used as an index of thyroid secretion. Under ATD, TSH and Tg levels were respectively: 35 mIU/l and 670.5 pmol/l (patient 1), 87 mIU/l and 453 pmol/l (patient 2) and 110 mIU/l and 906 pmol/l (patient 3). When DT4 was added (patient 1, 3 mg daily; patients 2 and 3, 2 mg daily) to the same dose of ATD, plasma TSH and Tg levels fell but were still over the upper limit of normal and thyrotoxicosis persisted as illustrated by a recurrence of atrial fibrillation in one patient. When ATD were withdrawn and DT4 given alone (2 mg daily) all symptoms subsided within 1 month while TSH and Tg levels fell within the normal range. TSH normalization was documented within 1 week in one patient.(ABSTRACT TRUNCATED AT 250 WORDS)