A Diagnostic Pitfall: Acute Myeloid Leukemia Mimicking Methimazole-induced Agranulocytosis

Methimazole is widely used for treating Graves' disease and it is a well-recognized cause of agranulocytosis. However, this assumption may delay the diagnosis of underlying hematologic disorders. A woman in her early 50s with Graves' disease had been treated with methimazole for 24 years. Routine laboratory testing revealed neutropenia, accompanied by mild anemia and thrombocytopenia. Despite the withdrawal of methimazole, cytopenia persisted. One month later, circulating blasts appeared, and a diagnosis of acute myeloid leukemia was made based on a bone marrow examination. Persistent cytopenia after the discontinuation of methimazole, particularly with multilineage involvement, should prompt an early evaluation for bone marrow disorders.