The
antithyroid drugs mainly include thioimidazole (
carbimazole,
methimazole=
thiamazole) and
propylthiouracil. After absorption,
carbimazole is rapidly metabolized to
methimazole and thus switching between these two drugs should not be considered in case of side effects. Furthermore, in case of side effects, sometimes even cross reactions between thioimidazoles and
propylthiouracil occur. Common and typical adverse reactions of
antithyroid drugs include dose dependent
hypothyroidism and thus thyroid function should be repeatedly checked while the patient is on
antithyroid drugs. Furthermore,
pruritus and
rash may develop. In this case, one might try to switch from thioimidazoles to
propylthiouracil or vice versa.
Antithyroid drugs may cause mild dose dependent
neutropenia or severe
allergy-mediated
agranulocytosis, which typically occurs during the first three months of treatment, has an incidence of 3 per 10,000 patients and cross reactivity between thioimidazoles to
propylthiouracil may occur. Rarely,
antithyroid drugs can cause
aplastic anemia. Mainly
propylthiouracil, but sometimes also
methimazole may lead to an asymptomatic transient increase in liver
enzymes or to severe, even lethal liver injury of cholestatic or hepatocellular pattern. Since
propylthiouracil associated liver injury was observed increasingly among children and adolescent, it has been suggested to prefer thioimidazoles for these patients. Because of these potential serious adverse effects, physicians should advise patients to immediately seek medical help if they get a
fever or
sore throat or malaise, abdominal complaints or
jaundice, respectively. Furthermore,
arthralgias may develop in 1-5% of patients under both
antithyroid drugs. Since
arthralgias may be the first symptom of more serious immunologic side effects, it is recommended to stop the
antithyroid drug in this case.
Drug induced
polyarthritis mainly develops during the first month of
therapy, whereas
ANCA-positive
vasculitis is generally observed only after long term exposure to
propylthiouracil or very rarely with the thioimidazoles. The teratogenic risk of the thioimidazoles is somewhat higher (
Aplasia cutis congenita), that is why one generally recommends preferring
propylthiouracil during pregnancy. During breast feeding both, thioimidazoles or
propylthiouracil, may be administered. Nowadays,
perchlorate is only used short term in case of latent
hyperthyroidism before administering
iodine-containing
contrast agents. Therefore, the known side effects, which usually are only observed after long term treatment, are not an issue any more.