Graves' Masquerade: A Case of Resistance to Thyroid Hormone (RTH) Syndrome

Resistance to thyroid hormone (RTH) syndrome is characterized by reduced sensitivity to thyroid hormones (TH). It is an autosomal dominant genetic disease commonly caused by a mutation of the thyroid hormone receptor beta (THR-β) gene. Manifestations of RTH can be variable, as one can be clinically euthyroid to hyperthyroid or hypothyroid. We would like to share a case of resistance to thyroid hormone beta (RTH β) that presented with atrial fibrillation (AF) and was treated as Graves' disease with antithyroid drugs initially but was otherwise clinically euthyroid. The diagnosis was subsequently revised to RTH after a delay of more than eight years when the patient was referred to endocrinology for an aberrant thyroid function test with persistently raised TH and an unsuppressed thyrotropin level after performing the appropriate investigations. This case illustrates the challenge in diagnosing RTH in individuals with apparently preserved cardiac tissue TH sensitivity mimicking Graves' disease and the importance of recognition of RTH to avoid inappropriate therapy to suppress the compensatory increase in TH production to maintain the euthyroid state in affected individuals.