Supplementary Material for: Methimazole induced agranulocytosis - A clinical case report

Antithyroid drugs are generally safe. However, rare but life-threatening side effects, such as agranulocytosis, may occur.
We present the case of a 70-year-old woman with subclinical hyperthyroidism, previously treated with propylthiouracil which was discontinued due to an unspecified mild white blood cell alteration. On presentation to our clinic, she reported anxiety with no other symptoms and a low dose methimazole (2.5mg/daily) was started. After two months, her white blood cell count was decreased. Methimazole was suspended pending further testing. Six days later, she presented to the emergency room with pharyngitis and fever. Broad-spectrum antibiotics were started and granulocyte-colony stimulating factor was administered following a diagnosis of methimazole induced agranulocytosis.
The onset of agranulocytosis is sudden, and patients should be educated about warning signs. If granulocytopenia is confirmed, medication should be discontinued. Switching to another antithyroid drug is not recommended, because a cross-reactivity between propylthiouracil and methimazole has been observed.