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Methimazole Pregnancy and Breastfeeding Warnings

Medically reviewed by Last updated on Aug 14, 2023.

Methimazole is also known as: Northyx, Tapazole

Methimazole Pregnancy Warnings

Benefit should outweigh risk

US FDA pregnancy category: D

Risk Summary: This drug crosses the placental membrane and can cause fetal harm, especially during the first trimester; studies have shown that the incidence of congenital malformations is greater in babies of mothers whose hyperthyroidism has remained untreated than in those who have been treated with anti-thyroid medication.

-If a patient is, or becomes pregnant during therapy, she should be advised of the potential risks to the fetus; women of childbearing potential should discuss contraception with their healthcare provider.
-Because congenital malformations have been associated with use of this drug, alternative anti-thyroid therapy is generally recommended during the first trimester; a switch to this drug is often recommended during the second and third trimester of pregnancy due to the potential of hepatotoxicity with propylthiouracil.
-The lowest effective dose should be used; thyroid dysfunction often diminishes as pregnancy proceeds and dose reduction or discontinuation of therapy may be possible several weeks or months before delivery.

This drug has been associated with congenital malformations. Cases of renal, skull, cardiovascular congenital defects, exomphalos, gastrointestinal malformation, umbilical malformations and duodenal atresia have been reported. A causal relationship of these malformations, especially choanal atresia and aplasia cutis congenital (congenital scalp defects), to this drug cannot be excluded. As this drug crosses the placenta, it can induce goiter and cretinism in the developing fetus; therefore, the lowest possible dose should be used. There are no controlled data in human pregnancy.

US FDA pregnancy category D: There is positive evidence of human fetal risk based on adverse reaction data from investigational or marketing experience or studies in humans, but potential benefits may warrant use of the drug in pregnant women despite potential risks.

See references

Methimazole Breastfeeding Warnings

Maternal doses up to 20 mg per day do not appear to affect thyroid function or intellectual development of breastfed infants. The American Thyroid Association does not recommend routine assessment of serum thyroid function in breastfed children; appropriate growth and development should be monitored during routine pediatric health and wellness evaluations. Agranulocytosis may occur; a complete blood count and differential should be obtained if there is a suspicion of a drug-induced blood dyscrasia.

This drug has been used without apparent harmful effects

Excreted into human milk: Yes

-Negative clinical effects on breastfed infant have not been observed, especially when maternal thyroid function is monitored frequently (week or biweekly)
-Infant dose can be minimized by waiting 3 to 4 hours after taking this drug to breastfeed.

See references

References for pregnancy information

  1. Product Information. Tapazole (methimazole). King Pharmaceuticals Inc. 2001.

References for breastfeeding information

  1. Product Information. Tapazole (methimazole). King Pharmaceuticals Inc. 2001.
  2. United States National Library of Medicine. Toxnet. Toxicology Data Network. 2013.

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.