Patients who receive methimazole should be under close surveillance and should be cautioned to report
immediately any evidence of illness, particularly sore throat, skin eruptions, fever, headache, or general
malaise. In such cases, white-blood-cell and differential counts should be obtained to determine
whether agranulocytosis has developed. Particular care should be exercised with patients who are
receiving additional drugs known to cause agranulocytosis.
Because methimazole may cause hypoprothrombinemia and bleeding, prothrombin time should be
monitored during therapy with the drug, especially before surgical procedures. Thyroid function tests
should be monitored periodically during therapy. Once clinical evidence of hyperthyroidism has
resolved, the finding of a rising serum TSH indicates that a lower maintenance dose of TAPAZOLE
should be employed.
Carcinogenesis, Mutagenesis, Impairment Of Fertility
In a 2 year study, rats were given methimazole at doses of 0.5, 3, and 18 mg/kg/day. These doses were
0.3, 2, and 12 times the 15 mg/day maximum human maintenance dose (when calculated on the basis of
surface area). Thyroid hyperplasia, adenoma, and carcinoma developed in rats at the two higher doses.
The clinical significance of these findings is unclear.
Pregnancy Category D
If TAPAZOLE is used during the first trimester of pregnancy or if the patient becomes pregnant while
taking this drug, the patient should be warned of the potential hazard to the fetus.
In pregnant women with untreated or inadequately treated Graves' disease, there is an increased risk of
adverse events of maternal heart failure, spontaneous abortion, preterm birth, stillbirth and fetal or
Because methimazole crosses placental membranes and can induce goiter and cretinism in the
developing fetus, hyperthyroidism should be closely monitored in pregnant women and treatment
adjusted such that a sufficient, but not excessive, dose be given during pregnancy. In many pregnant
women, the thyroid dysfunction diminishes as the pregnancy proceeds; consequently, a reduction of
dosage may be possible. In some instances, anti-thyroid therapy can be discontinued several weeks or
months before delivery.
Due to the rare occurrence of congenital malformations associated with methimazole use, it may be
appropriate to use an alternative anti-thyroid medication in pregnant women requiring treatment for
hyperthyroidism particularly in the first trimester of pregnancy during organogenesis.
Given the potential maternal adverse effects of propylthiouracil (e.g., hepatotoxicity), it may be
preferable to switch from propylthiouracil to TAPAZOLE for the second and third trimesters.
Methimazole is present in breast milk. However, several studies found no effect on clinical status in
nursing infants of mothers taking methimazole. A long-term study of 139 thyrotoxic lactating mothers
and their infants failed to demonstrate toxicity in infants who are nursed by mothers receiving treatment
with methimazole. Monitor thyroid function at frequent (weekly or biweekly) intervals.
Because of postmarketing reports of severe liver injury in pediatric patient treated with
propylthiouracil, TAPAZOLE is the preferred choice when an antithyroid drug is required for a
pediatric patient (see DOSAGE AND ADMINISTRATION).