Methimazole

The Basics

What is Methimazole?

Used to treat hyperthyroidism.

Brand names for Methimazole

Tapazole

How Methimazole is classified

Antithyroid Agents, Thionamides

Methimazole During Pregnancy

Methimazole pregnancy category

Category DNote that the FDA has deprecated the use of pregnancy categories, so for some medications, this information isn’t available. We still think it’s useful to list historical info, however, given what a common proxy this has been in the past.

What we know about taking Methimazole while pregnant

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 neonatal hyperthyroidism. 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.

Taking Methimazole While Breastfeeding

What are recommendations for lactation if you're taking Methimazole?

Maternal methimazole therapy does not affect thyroid function or intellectual development in breastfed infants with doses up to 20 mg daily. Taking methimazole right after nursing and waiting for 3 to 4 hours before nursing again should minimize the infant dosage. No cases of thyroid function alteration have been reported among infants exposed to methimazole via breastmilk. The American Thyroid Association recommends only monitoring infants for appropriate growth and development during routine pediatric health and wellness evaluations and routine assessment of serum thyroid function in the child is not recommended.[1] Rare idiosyncratic reactions (e.g., agranulocytosis) might occur, and the infant should be watched for signs of infection. Monitoring of the infant’s complete blood count and differential is advisable if there is a suspicion of a drug-induced blood dyscrasia.

Maternal / infant drug levels

Maternal methimazole therapy does not affect thyroid function or intellectual development in breastfed infants with doses up to 20 mg daily. Taking methimazole right after nursing and waiting for 3 to 4 hours before nursing again should minimize the infant dosage. No cases of thyroid function alteration have been reported among infants exposed to methimazole via breastmilk. The American Thyroid Association recommends only monitoring infants for appropriate growth and development during routine pediatric health and wellness evaluations and routine assessment of serum thyroid function in the child is not recommended.[1] Rare idiosyncratic reactions (e.g., agranulocytosis) might occur, and the infant should be watched for signs of infection. Monitoring of the infant’s complete blood count and differential is advisable if there is a suspicion of a drug-induced blood dyscrasia.

Possible effects of Methimazole on milk supply

Relevant published information was not found as of the revision date.

Possible alternatives to Methimazole

Propylthiouracil.