Treatment of resistance to thyroid hormone--primum non nocere

Thyroid hormone resistance and large goiter mimicking infiltrative carcinoma in a pediatric patient

OBJECTIVES

Resistance to thyroid hormone (RTH) is a genetic condition, caused by mutations in the thyroid hormone receptor gene and characterized by impaired end organ responsiveness to thyroid hormone. Here we describe a novel case of THR associated with large goiter mimicking infiltrative c.

CASE PRESENTATION

A 13-year-old male with a hyperthyroid phenotype of RTH diagnosed as a toddler, on methimazole and nadolol therapies presented with an increase in goiter size and possible nodule. Thyroid ultrasound was concerning for a diffuse infiltrative process or malignancy. Methimazole was discontinued and he underwent further imaging, fine needle aspiration and core biopsies. Biopsy results were reassuring and imaging findings were subsequently attributed to RTH rather than malignancy. He started every other day liothyronine therapy, which led to a decrease in goiter size, thyroglobulin level, and improvement of hyperthyroid symptoms.

CONCLUSIONS

This is the first case to our knowledge describing the above thyroid imaging findings in association with RTH. It also adds important information to the pediatric literature regarding management of the hyperthyroid phenotype of RTH, including the role of liothyronine therapy.

Assessing the clinical and molecular diagnosis of inherited forms of impaired sensitivity to thyroid hormone from a single tertiary center

AIM

Resistance to thyroid hormone (RTH), characterized by persistent hyperthyroxinemia with non-suppressed thyrotropin (TSH), is mostly caused by mutations in thyroid hormone receptor beta gene (THRB). Two differential diagnoses should be considered due to similar clinical and laboratory findings: TSH-producing pituitary adenoma (TPA) and Familial Dysalbuminemic Hyperthyroxinemia (FDH). The aim of this study is to describe our single tertiary center experience in the molecular diagnosis of RTH in Brazilian patients, analyzing their clinical and laboratory characteristics and the most common differential diagnosis.

SUBJECTS AND METHODS

We enrolled 30 subjects with clinical and laboratory features of RTH. Patient´s evaluations included clinical examination, thyroid hormone profile and imaging tests. Sequencing analysis for THRB hot spot region was conducted on all patients, and those without mutations in beta isoform of the thyroid hormone receptor (TRβ) (non-TR-RTH) were investigated for albumin gene (ALB) mutation.

RESULTS

Seventeen patients presented mutations in TRβ (RTHβ); six were non-TR-RTH, three had a diagnosis of FDH with a mutation in ALB, and four were diagnosed with TPA. Two characteristics were different to what is commonly described in the literature: higher serum TSH levels in RTHβ patients when compared to the non-TR-RTH group, but this difference did not extend to free T4 (FT4) level; also the percentage of non-TR-RTH was higher than what was reported in other series.

CONCLUSION

In the present series, most cases were RTHβ with higher levels of TSH. We described three novel mutations in THRB (p.M313V, p.R320G and p.R438P) and the first patients with FDH molecular diagnosis (p.R242H) documented in Brazil.

A clinician's guide to understanding resistance to thyroid hormone due to receptor mutations in the TRα and TRβ isoforms

There are two genes that express the major thyroid hormone receptor isoforms. Mutations in both these genes have given rise to Resistance to Thyroid Hormone (RTH) syndromes (RTHβ, RTHα) that can have variable phenotypes for mutations of the same receptor isoform as well as between the two receptor isoforms. In general, the relative tissue-specific distribution of TRβ and TRα determine RTH in different tissues for each form of RTH. These differences highlight some of the isoform-specific roles of each TR isoform. The diagnosis of RTH is challenging for the clinician but should be considered whenever a patient presents with unexplained elevated serum free T₄ (fT₄) and unsuppressed TSH levels, as well as decreased serum free T₄/T₃ ratio. Here we provide a guide for the clinician to diagnose and treat both types of RTH.

Syndrome of Reduced Sensitivity to Thyroid Hormones: Two Case Reports and a Literature Review

Resistance to thyroid hormone (RTH) is an extremely rare dominantly inherited condition of impaired tissue responsiveness to thyroid hormone (TH). Most patients with RTH have mutations in the gene that encodes the β isoform of the receptor of thyroid hormone (THR-β gene). Mutant receptors are unable to activate or repress target genes. The majority of them are asymptomatic or rarely have hypo- or hyperthyroidism. RTH is suspected by the finding of persistent elevation of serum levels of free T3 (FT3) and free T4 (FT4) and nonsuppressed TSH. We present two cases of RTH diagnosed after total thyroidectomy. The first patient was initially diagnosed with primary hyperthyroidism due to toxic multinodular goiter. The second patient had undergone thyroidectomy for multinodular goiter 16 years before diagnosis of RTH. After thyroidectomy, although on relatively high doses of levothyroxine, both of them presented with the laboratory findings of RTH. Genetic analysis revealed RTH.

Thyroid hormone resistance and its management

The syndrome of impaired sensitivity to thyroid hormone, also known as syndrome of thyroid hormone resistance, is an inherited condition that occurs in 1 of 40,000 live births characterized by a reduced responsiveness of target tissues to thyroid hormone due to mutations on the thyroid hormone receptor. Patients can present with symptoms of hyperthyroidism or hypothyroidism. They usually have elevated thyroid hormones and a normal or elevated thyroid-stimulating hormone level. Due to their nonspecific symptomatic presentation, these patients can be misdiagnosed if the primary care physician is not familiar with the condition. This can result in frustration for the patient and sometimes unnecessary invasive treatment such as radioactive iodine ablation, as in the case presented herein.

Periodic Paralysis as a New Phenotype of Resistance to Thyroid Hormone Syndrome in a Chinese Male Adult

CONTEXT

A majority of patients with resistance to thyroid hormone (RTH) are asymptomatic, whereas some patients show signs of hyperthyroidism, or hypothyroidism, or both. Thyrotoxic periodic paralysis is the most common form of acquired periodic paralysis. However, it has not been reported in a patient with RTH up to now.

OBJECTIVE

We evaluated a 36-year-old male patient from China with elevated serum free T4 and free T3 and inappropriately high TSH who presented with periodic paralysis.

STUDY DESIGN

Clinical, biochemical, and radiological assessments, as well as DNA sequencing, were performed.

RESULTS

The patient's laboratory tests revealed the following: TSH, 6.14 mIU/L (0.27-4.2 mIU/L); free T3, 12.85 pmol/L (2.8-7.1 pmol/L); free T4, 33.62 pmol/L (9.05-25.5 pmol/L); and serum SHBG, 19.4 nmol/L (18.3-54.1 nmol/L). No significant suppression of TSH was observed in the rapid TSH suppression test with somatostatin analogs. Compound muscle action potential after exercise of the patient was reduced by 58%. Sequencing of thyroid hormone receptor genes confirmed a C446S mutation in the THRβ gene.

CONCLUSIONS

This is the first report of periodic paralysis as a new phenotype of RTH syndrome.

Role and Mechanisms of Actions of Thyroid Hormone on the Skeletal Development

The importance of the thyroid hormone axis in the regulation of skeletal growth and maintenance has been well established from clinical studies involving patients with mutations in proteins that regulate synthesis and/or actions of thyroid hormone. Data from genetic mouse models involving disruption and overexpression of components of the thyroid hormone axis also provide direct support for a key role for thyroid hormone in the regulation of bone metabolism. Thyroid hormone regulates proliferation and/or differentiated actions of multiple cell types in bone including chondrocytes, osteoblasts and osteoclasts. Thyroid hormone effects on the target cells are mediated via ligand-inducible nuclear receptors/transcription factors, thyroid hormone receptor (TR) α and β, of which TRα seems to be critically important in regulating bone cell functions. In terms of mechanisms for thyroid hormone action, studies suggest that thyroid hormone regulates a number of key growth factor signaling pathways including insulin-like growth factor-I, parathyroid hormone related protein, fibroblast growth factor, Indian hedgehog and Wnt to influence skeletal growth. In this review we describe findings from various genetic mouse models and clinical mutations of thyroid hormone signaling related mutations in humans that pertain to the role and mechanism of action of thyroid hormone in the regulation of skeletal growth and maintenance.

Coexistence of THRB and TBG gene mutations in a Turkish family

OBJECTIVE

Resistance to thyroid hormone is a syndrome characterized by high serum free T4 levels and unsuppressed serum TSH concentration. Thyroxine-binding globulin complete deficiency manifests with low serum total T4 and T3 levels and normal serum TSH concentration. Our objective is to describe a family with the coexistence of resistance to thyroid hormone and thyroxine-binding globulin complete deficiency.

METHODS

We conducted clinical studies and genetic analyses.

RESULTS

The proband presented with mental retardation, hearing loss, and recurrent upper respiratory tract infections accompanied by high serum levels of TSH, T3, T4, and high thyroglobulin antibody titers. His elder sister presented with normal TSH and T3 and high serum T4 levels. Both patients were found to be heterozygous for the mutation P453A in the thyroid hormone receptor beta (THRB) gene. One of the proband's brothers had low serum total T3 and T4 and normal TSH concentrations, without any clinical manifestations. He was hemizygous for the mutation P50fs51X in the TBG gene. The proband's mother showed slightly elevated TSH, normal total T3 and T4, and elevated titers of thyroperoxidase antibodies and thyroglobulin antibodies. She was heterozygous for both THRB and TBG genes mutations.

CONCLUSIONS

To our knowledge, this is the first report of the coexistence of THRB and TBG gene mutations in the same individual (mother of the proband), whereas other affected family members had only 1 of the 2 genes mutated. The case illustrates the difficulty that might be encountered in the interpretation of thyroid function tests when different genetic defects affecting thyroid function coexist.

A mechanism for pituitary-resistance to thyroid hormone (PRTH) syndrome: a loss in cooperative coactivator contacts by thyroid hormone receptor (TR)beta2

Thyroid hormone receptors (TR) are hormone-modulated transcription factors that regulate overall metabolic rate, lipid utilization, heart rate, and development. TR are expressed as a mix of interrelated receptor isoforms. The TRβ2 isoform is expressed in the hypothalamus and pituitary, where it plays an important role in the feedback regulation of thyroid hormone levels. TRβ2 exhibits unique transcriptional properties that parallel the ability of this isoform to bind to certain coactivators cooperatively through multiple contact surfaces. The more peripherally expressed TRβ1 isoform, in contrast, appears to recruit these coactivators through a single contact mechanism. We report here that clusters of charged amino acids in the TR hormone-binding domain are required for this enhanced mode of coactivator recruitment and that mutations in these charge clusters, by disrupting TRβ2 coactivator binding, are a molecular basis for pituitary resistance to thyroid hormone, a disease characterized by inappropriate thyroid hormone feedback regulation. We propose that the charge clusters allow wild-type TRβ2 to assume a conformation compatible with its mode of multiple contact coactivator recruitment, whereas disruption of these charge clusters disrupts normal T(3) homeostasis by reducing TRβ2 to a TRβ1-like, single contact mode of coactivator binding.

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.