Thyrotropin (TSH)-Binding Inhibiting Immunoglobulin-Negative, Thyroid-Stimulating Antibody-Positive Amiodarone-Induced Thyrotoxicosis.

Background/Objective: Amiodarone-induced thyrotoxicosis (AIT) is classified into type 1 (AIT1) and type 2 (AIT2) using several diagnostic procedures, including thyrotropin receptor antibodies (TRAbs). While thyrotropin-binding inhibiting immunoglobulin (TBII) and thyroid-stimulating antibody (TSAb) are widely used TRAb assays for diagnosing Graves' disease, their roles in distinguishing AIT subtypes remain unclear. Herein, we report a patient with TBII-negative, TSAb-positive AIT1. Case Report: A 32-year-old man with a history of ventricular fibrillation, who had been receiving amiodarone for 4 years, presented with diffuse goiter and palpitations. Endocrinologic evaluation revealed thyrotoxicosis with undetectable TBII levels, measured using an M22 human monoclonal TRAb assay. Although the patient was initially diagnosed with AIT2 and treated with prednisolone, his positive TSAb bioassay results and normal thyroid uptake on technetium-99m pertechnetate scintigraphy suggested AIT1. Prednisolone alone partially reduced thyroid hormone levels, whereas the addition of methimazole normalized thyroid function, indicating AIT1 due to underlying Graves' disease, possibly coexisting with AIT2. Discussion: This case highlights that TBII can be negative in AIT1 and can underscore the potential diagnostic utility of TSAb measurement. These findings suggest that TSAb measurement may serve as supportive evidence for diagnosing AIT1 in TBII-negative patients. Conclusion: We report a case of TBII-negative, TSAb-positive AIT1, highlighting the role of TSAb measurement in detecting AIT1, particularly in TBII-negative AIT patients.