Antithyroid arthritis syndrome caused by methimazole in a patient with Graves' disease.

Clinicians should be aware of the possibility of developing antithyroid arthritis syndrome (AAS) in patients treated with antithyroid medications, which can lead to severe migratory polyarthritis. Prompt cessation of the antithyroid agent is essential for the resolution of AAS. Anti-neutrophil cytoplasmic antibody (ANCA) negativity is needed to differentiate from antithyroid agent-induced ANCA-associated vasculitis, which shows arthritis similar to AAS.