Hyperthyroidism is an endocrine disorder characterized by hypermetabolism and increased excitability of multiple organs attributable to improper and excessive synthesis and the secretion of thyroid hormones. Methimazole (MMI), one of the first-line antithyroid drugs, is the preferred drug for hyperthyroidism therapy. 1 Management of Graves thyroidal and extrathyroidal disease: an update. Long-term antithyroid drug treatment is safe and causes adverse effects in only 3% to 5% of patients. , 3 Safety of long-term antithyroid drug treatment? a systematic review. , 4 Bartalena L Bogazzi F Martino E. Adverse effects of thyroid hormone preparations and antithyroid drugs. Common adverse reactions to this treatment include rash, itching, joint pain, and transient leukopenia, which are always mild and transient. , 3 Safety of long-term antithyroid drug treatment? a systematic review. , 4 Bartalena L Bogazzi F Martino E. Adverse effects of thyroid hormone preparations and antithyroid drugs. Other major adverse reactions include agranulocytosis, toxic hepatitis, vasculitis, and drug-induced lupus erythematosus. , 3 Safety of long-term antithyroid drug treatment? a systematic review. , 4 Bartalena L Bogazzi F Martino E. Adverse effects of thyroid hormone preparations and antithyroid drugs. Nevertheless, MMI-associated myocarditis was not mentioned in its drug assessment and was rarely reported in previous cases. We present a case of a young male patient who developed fever, chest discomfort, and joint symptoms after taking MMI. This patient provided clinical and laboratory information and agreed to the case publication with signed informed consent.