Failure to differentiate resistance to thyroid hormone (RTH) from primary thyrotoxicosis can result in inappropriate treatment which can potentially cause overt hypothyroidism. We report a case of RTH which was treated with methimazole.
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Case: 51 years old male with previous history of goiter status post right lobectomy (benign) 6 years ago was referred to endocrinology office for evaluation of abnormal thyroid function test. His yearly TSH remained normal since surgery. Recently free T4 (FT4) level was also checked and was high at 2.7 ng/dl. His only symptom was intermittent palpitations. He was started on low dose methimazole which resulted in elevated TSH at 6.3 mcIU/ml. Thyroid ultrasound and pituitary MRI were unremarkable. Pituitary hormone profile including alpha subunit of TSH were normal ruling out TSH secreting adenoma. A 24-hour thyroid uptake scan showed diffusely elevated uptake. Dose of methimazole was increased resulting in normalization of FT4 but TSH went markedly up at 48 mcIU/ml. Thyroid stimulating immunoglobulins were negative ruling out Graves’ disease. Sex hormone binding globulin was normal and serum total and FT4 by equilibrium dialysis were high ruling out thyroid hormone binding abnormalities. DNA studies showed p.Met313Thr mutation in the thyroid hormone receptor beta gene (THRB) which has deleterious effect on THRB protein function. His methimazole was stopped resulting in return of thyroid profile back to initial values. He was started on low dose propranolol for palpitations.
RTH is an inherited syndrome characterized by high FT4 and FT3 with normal or slightly high TSH. In most cases, RTH is caused by defect in the THRB. Resistance of pituitary thyrotropes to thyroid hormone raises TSH secretion, which increases the synthesis of T4 and T3 which in turn, fail to downregulate the hypothalamic-pituitary-thyroid axis. Goiter and tachycardia are the most common symptoms, when present. TSH secreted by these patients has increased bioactivity compared to normal TSH which explains goiter and high T4, T3 even with normal or slightly high TSH. The severity of resistance varies among different tissues due to differences in the relative expression of thyroid receptors (THRA and THRB). This may explain tachycardia due to high FT3 and FT4, as heart predominantly expresses THRA. As increased thyroid hormone compensates for thyroid hormone resistance, most patients are clinically euthyroid and do not need treatment.
Conclusion : Thyroid ablative or antithyroid treatment for erroneous diagnosis of hyperthyroidism, in patients with underlying thyroid hormone resistance can lead to overt hypothyroidism and should be avoided.
Wellspan York Hospital
I am an internal medicine resident at York Hospital. I am planning to pursue Endocrinology fellowship. I like to travel and take pictures.
Internal Medicine Resident at York Hospital. I will be starting my Endocrinology fellowship at University of Maryland next year. I have a special interest in Thyroid disorders.