Are Antithyroid Drugs Safe While Breastfeeding?

Mother breastfeeding baby in living room
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There are many known benefits of breastfeeding. According to the Centers for Disease Control, human milk is the easiest for babies to digest and includes hundreds of nutrients babies need in their first months of life. Breast milk also helps provide immunity to a number of illnesses, and being breastfed is associated with a lowered risk of obesity, diabetes, allergies, asthma, less risk of sudden infant death syndrome, ear infections, diarrhea, and gastrointestinal illnesses as infants, as well as higher IQ levels.

Breastfeeding may also help the nursing mother reduce her risk of ovarian and breast cancer, lose weight more quickly postpartum, and save money on formula costs. 

In hyperthyroidism, the thyroid gland becomes overactive and produces an excess of thyroid hormone. Hyperthyroidism — which is sometimes caused by the autoimmune condition Graves' disease — needs to be treated carefully during and after pregnancy. The same is true for women who have hyperthyroidism and Graves' disease or develop it while breastfeeding.

Some patients are on antithyroid drug treatment for their overactive thyroid condition. In the past, women taking antithyroid drugs for hyperthyroidism were discouraged from breastfeeding their newborns or infants. The concerns were centered on whether the antithyroid drugs taken by the mother, and passed on through breastmilk, would suppress the baby's thyroid gland, or cause an enlargement of the gland known as goiter.

Currently, the issue of breastfeeding while taking antithyroid drugs remains somewhat controversial. Can a woman taking antithyroid drugs like methimazole (Tapazole), carbimazole, or propylthiouracil (PTU) to treat her hyperthyroidism or Graves' disease safely breastfeed her newborn or infant?

In general, the experts say yes. Doctors recommend that new mothers on antithyroid drugs breastfeed — or not be discouraged from breastfeeding — because the benefits of breastfeeding are documented, and the exposure of babies to the antithyroid drugs that pass into the breast milk is considered "minimal and not clinically significant."  

Taking Antithyroid Drugs While Breastfeeding

No medical experts have said that any antithyroid drug is 100% safe for use while breastfeeding. 

As far as safety, however, a number of studies have found no alteration in thyroid function, or effects on thyroid function and physical and intellectual development in infants who are breastfed by mothers treated with daily doses of PTU of up to 300 mg a day, methimazole at doses of up to 30 mg a day, and carbimazole at doses of up to 15 mg) for as long as eight months. 

According to the European Thyroid Journal, which published an article on the topic in 2012: 

  • PTU should not be prescribed while a woman is breastfeeding — PTU is linked to a small risk of liver damage — and instead, methimazole (up to 30 mg/day) or carbimazole (up to 15 mg/day) can be used. (PTU is only recommended for restricted time periods, in special cases, but it is safe for a nursing infant as long as the dosage does not exceed 300 mg a day.)
  • Careful monitoring of both mother and infant is still advisable, including T4 and TSH levels, at least 3 to 4 weeks after starting breastfeeding.

A special note: Most experts recommend that a nursing mother breastfeed before taking her antithyroid medications, to minimize the baby's exposure to the drug. 

Ultimately, the decision to breastfeed while taking antithyroid medications is one a woman should make in conjunction with her own healthcare provider, and her infant's pediatrician.

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Article Sources
  • Glatstein, MD, Miguel Marcelo et. al. "Pharmacologic treatment of hyperthyroidism during lactation," Canadian Family Physician. 2009 Aug; 55(8): 797–798. PMCID: PMC2726094
  • Karras, Spiros  and Krassas, Gerasimos "Breastfeeding and Antithyroid Drugs: A View from Within." Eur Thyroid J. 2012 Apr; 1(1): 30–33. Published online 2012 Feb 29. doi:  10.1159/000336595 PMCID: PMC3821454