Three cases of methimazole-induced fever in patients with Graves’ hyperthyroidism

Methimazole (MMI), a widely used antithyroid drug (ATD), is generally well tolerated, but adverse effects such as agranulocytosis may occur. We report three cases of MMI-induced fever in patients with newly diagnosed Graves’ hyperthyroidism. All patients developed fever within 11–12 days of MMI initiation, but without neutropenia and other major adverse events. The fevers resolved within 48 hours of MMI withdrawal, supporting the diagnosis of drug-induced fever. In all three cases, initial physical and laboratory findings were non-specific, and patients appeared clinically well despite having high-grade fever. Potassium iodide (KI) monotherapy was used as interim therapy and was effective in maintaining euthyroidism without recurrence of fever. Two patients were subsequently switched to propylthiouracil (PTU), with one developing a cutaneous drug eruption but no recurrence of fever. Although rare, ATD-induced fever should be considered in differential diagnosis when fever arises, especially in the absence of neutropenia or elevated inflammatory markers. Prompt recognition and discontinuation of the offending agent are essential for preventing misdiagnosis and inappropriate treatment. KI monotherapy can serve as a safe and effective bridging therapy in such cases. Switching to alternative ATDs may be acceptable when definitive treatment such as radioactive iodine or surgery is not feasible, though careful monitoring is warranted due to risks of new adverse events. To our knowledge, this is the first English-language case series to provide detailed clinical descriptions of three consecutive cases of MMI-induced fever which highlights practical strategies for diagnosis and management.