Methimazole-Induced Antithyroid Arthritis Syndrome in Graves' Disease Management: A Case Report

BACKGROUND Antithyroid arthritis syndrome (AAS) is a rare adverse reaction to antithyroid drugs (ATDs), characterized by fever, rash, myalgia, and migratory polyarthralgia. It mimics rheumatic disorders, complicating diagnosis. CASE REPORT We present a case with unique symptoms presentation of a 29-year-old Chinese man with Graves' disease who presented with fever, rash and myalgia following methimazole initiation, then developed systemic migratory pain in muscle and joints after stopping medication. Initial suspicion of ANCA-associated vasculitis (AAV) or drug-induced lupus arose due to elevated inflammatory markers and negative ANCA/autoimmune serology (except ANA+). Pharmacist-led assessment using the Naranjo scale and the inquiry during daily care confirmed the temporal association with methimazole. After excluding AAV and lupus via clinical and laboratory criteria, AAS was diagnosed. Corticosteroids and lithium carbonate was prescribed for the pain and Graves' disease control at admission, then necessitating dosage adjustment for overall safety. Propylthiouracil was prescribed during a subsequent visit, but he then developed drug-induced liver injury manifested as jaundice, which finally required management with radioiodine therapy, which led to control of symptoms. CONCLUSIONS This case highlights the diagnostic complexity of AAS, particularly in male patients, and underscores the therapeutic role of lithium carbonate as a transitional agent for thionamide-intolerant Graves' disease. Given the high cross-sensitivity between antithyroid drugs limiting pharmacological alternatives, propylthiouracil also carries a significant risk of adverse reactions. Pharmacist involvement in adverse drug reaction assessment and lithium dosing optimization was critical for successful management. Heightened clinical vigilance and experience are essential when encountering suspected AAS.