Antithyroid drugs are a mainstay of therapy of hyperthyroidism, especially
when it is due to Graves' disease. Although antithyroid agents are generall
y safe, side effects may limit their usefulness in some patients. Side effe
cts are classified as minor and major, based on their degree of morbidity.
All side effects are more common at higher doses of methimazole (40 mg/day
or more), but there is no apparent dose relationship for propylthiouracil (
PTU). The most common minor side effects include cutaneous eruptions, arthr
algias, and gastrointestinal upset, which develop in up to 5% of patients a
t usual doses. Hair loss, sialadenitis, myopathy, and abnormalities of tast
e and smell also have been described. The common major side effects are agr
anulocytosis (frequency 0.1-0.4%) and frank polyarthritis. Severe systemic
vasculitis or drug-induced lupus (often antineutrophil cytoplasmic antibody
positive) are associated with PTU. Hepatic toxicity with methimazole is ch
olestatic in nature and not as severe as the potentially life threatening h
epatocellular reactions that are seen with PTU. Hypoprothrombinemia and the
insulin autoimmune syndrome are exceedingly rare major side effects.