Which antithyroid drug is most commonly associated with agranulocytosis and requires CBC monitoring?

• A. Methimazole
• B. Propylthiouracil
• C. Carbimazole
• D. Radioiodine therapy

Answer: (A)

Agranulocytosis is a rare but serious adverse effect of thionamide antithyroid drugs, where neutrophil counts plummet and infections can occur. Because this risk can present abruptly, baseline and periodic complete blood counts are recommended, especially during the first few months of therapy, and patients are advised to watch for fever or sore throat.

Methimazole is the antithyroid drug most commonly associated with agranulocytosis, so CBC monitoring is particularly emphasized with its use. Propylthiouracil can also cause this complication, but methimazole is the one most frequently linked in clinical practice and guidelines. Carbimazole has a similar risk profile. Radioiodine therapy is not an antithyroid drug and does not carry the same agranulocytosis monitoring requirement.