6415 Resistance to Thyroid Hormone -A Challenge to Manage

Abstract Disclosure: A. Altaf: None. F. Iqbal: None. E. Hamoudeh: None. C. Coyne: None. K. Anne: None. L. Buckley: None. Background: Resistance to thyroid hormone (RTH) or Refetoff syndrome was first introduced in 1967. It is an autosomal dominant or recessive genetic disease with a mutation in the thyroid hormone receptorβ (THR-β) gene or the thyroid hormone receptorα (THR-α) gene. The distinguishing biochemical feature of RTHβ is an elevated level of circulating free thyroid hormones with normal or high TSH concentration and decreased peripheral tissue response to thyroid hormone. Common signs and symptoms in patients with RTHβ are short stature, attention deficit disorder, and resting tachycardia and many patients might be asymptomatic. Case: We report a case of a 36-year-old patient with past medical history of congenital bilateral hearing loss, atrial fibrillation, CHF, and slow learning/comprehending who presented to the ER with shortness of breath and atrial fibrillation with tachycardia. Admission labs revealed elevated TSH 3.9, elevated fT4 3.28, and elevated TT3 205. He reported that for the past year, he has been on levothyroxine 50mcg for abnormal TSH. Levothyroxine was stopped and the patient was discharged on methimazole 10mg daily. Further workup revealed negative thyroglobulin and thyroid peroxidase antibodies. Thyroid ultrasound with Doppler demonstrated a mildly hypervascular thyroid gland without evidence of nodules. Brain MRI was negative for any pituitary abnormalities. HAMA testing continued to show elevated TSH. Patient was followed by cardiology and due to continued atrial fibrillation, had multiple cardioversions and was managed on a beta blocker. Due to his atrial fibrillation, endocrinology service continued his methimazole till his genetic testing came positive for an autosomal dominant heterozygous pathogenic mutation in THRB c.1373 T>C (p. Val458Ala). His thyroid hormones and TSH continue to be elevated. Currently, he is not on thyroid medication and getting monitored. The need for multiple cardioversions for atrial fibrillation in this patient makes resistance to thyroid hormone management challenging. Discussion: Hyperthyroidism is commonly mistaken for RTH. However, the lack of TSH suppression should be further investigated. Most patients with RTHβ are adequately compensated by the increased endogenous supply of thyroid hormone showing increased serum fT3 and fT4 levels and normal or near normal TSH levels. Beta Blockers like atenolol cannot inhibit the conversion of T4 to T3 and can be used for RTHβ with tachycardia. Triiodothyroaceticacid (TRIAC) or dextrothyroxine (DT4) have also been used to treat thyrotoxicosis. In summary, treatment for RTHβ remains a challenge for clinicians, especially in patients with atrial fibrillation and many patients are misdiagnosed and managed as hyperthyroidism. Presentation: 6/3/2024