Adverse effects of antithyroid drugs

Antithyroid drugs can rarely cause agranulocytosis; it is most likely to occur in the first months of therapy. Advise patients to immediately stop the drug and to seek medical assessment if they experience acute malaise, fever or infection (typically severe pharyngitis).

Do not use carbimazole or propylthiouracil in a patient with a history of agranulocytosis caused by either drug. Consider other options to treat hyperthyroidism (eg thyroidectomy, radioiodine).

Propylthiouracil has been associated with rare cases of severe liver injury causing death or requiring liver transplantation. It is contraindicated in children. Advise adults taking propylthiouracil to immediately stop the drug and to seek medical assessment if they experience malaise, anorexia, nausea, abdominal pain, discoloured urine or jaundice.

Vasculitis with positive antineutrophil cytoplasmic antibodies (ANCA) is a rare complication of propylthiouracil treatment. Seek specialist advice if this occurs.