Two Cases of Methimazole-Induced Agranulocytosis With Their Risk Factors

Abstract Background Antithyroid drugs, such as methimazole (MMI), are standard therapies for the medical management of thyrotoxicosis. Agranulocytosis is a rare but lethal adverse effect of antithyroid medications. We have reported 2 cases of MMI-induced agranulocytosis with similar risk factors that likely predisposed them to this adverse reaction. Case Report Case 1 involved a 71-year-old woman, with a history of Graves disease, who presented with an altered mental status. She was recently discharged on 40 mg of MMI twice daily, and she continued this dose for 2 months. She was readmitted and found to have neutropenic fever in the setting of MMI-induced agranulocytosis. MMI was discontinued, and she was started on filgrastim. Her cell counts gradually improved, and she was subsequently discharged. Case 2 involved a 68-year-old woman, with a history of Graves disease, who presented with severe back pain, nausea, and vomiting. She was recently discharged on 10 mg of MMI twice daily, which was increased to 10 mg 3 times a day. She was readmitted to the hospital because of a septic shock in the setting of pneumonia, colitis, bacteremia, and MMI-induced agranulocytosis. A bone marrow biopsy showed a polyclonal infiltrate with up to 85% plasma cells. Despite treatment with antibiotics, filgrastim, and continuous renal replacement therapy, she ultimately passed away. Discussion Although these cases had differing outcomes, they shared similar features and risk factors, including older age, female sex, and relatively higher doses of MMI. Conclusion Close follow up and awareness of risk factors, such as age, female sex, and higher doses of MMI, may decrease the risk of MMI-induced agranulocytosis and fatal outcomes.