Thyroid Hormone Resistance Syndrome: A Case Report with Literature Review

Abstract

Resistance to thyroid hormone receptor beta (THRβ) is a rare condition causing abnormal thyroid function tests (TFTs) characterized by elevated thyroid hormone levels with unsuppressed Thyroid Stimulating Hormone (TSH). Thyroid hormone action involves multiple steps, and mutations affecting these steps are key to understanding and managing thyroid disorders. We present a case of THRβ resistance associated with cardiac arrhythmia. A 40-year-old male with a history of atrial fibrillation (AF) was referred for evaluation of abnormal TFTs and thyroid nodules. TFTs revealed a normal TSH and elevated free thyroxine. Imaging showed a large, peripherally enhancing necrotic mass with calcification in the left thyroid lobe and a 0.8 cm hypodense area in the right lobe. Thyroid ultrasound confirmed bilateral nodules, with the largest in the lower pole of the left lobe. The fine-needle aspiration biopsy was benign (Bethesda category II). Inherited THRβ pathogenic variants cause thyroid hormone resistance, often resulting in an enlarged thyroid gland. Despite this resistance, patients may still show clinical signs of cardiac arrhythmias. Diagnosing thyroid hormone resistance helps avoid unnecessary treatment for asymptomatic patients.