This sheet is about exposure to methimazole in pregnancy and while breastfeeding. This information is based on available published literature. It should not take the place of medical care and advice from your healthcare provider.

What is methimazole?

Methimazole is a medication that has been used to treat hyperthyroidism (when the thyroid gland makes too much thyroid hormone) and Graves’ disease (a common cause of hyperthyroidism). Methimazole lowers the amount of thyroid hormone that the thyroid gland makes. Tapazole® was a brand name for methimazole.

Sometimes when women find out they are pregnant, they think about changing how they take their medication, or stopping their medication altogether. However, it is important to talk with your healthcare providers before making any changes to how you take your medication. Untreated hyperthyroidism can increase the chance of illness for the woman who is pregnant and the chance of pregnancy complications. Your healthcare providers can talk with you about the benefits of treating your condition and the risks of untreated illness during pregnancy.

The US Food and Drug Administration (FDA), The American College of Obstetricians and Gynecologists (ACOG), and The American Thyroid Association (ATA) have stated that propylthiouracil (PTU), another medication that has been used to treat hyperthyroidism, might be the preferred treatment for hyperthyroidism during the first trimester of pregnancy. MotherToBaby has a fact sheet on PTU here: https://mothertobaby.org/fact-sheets/propylthiouracil-ptu/.

I take methimazole. Can it make it harder for me to get pregnant?

Studies have not been done to see if methimazole can make it harder to get pregnant. Untreated thyroid disorders can make it harder to get pregnant.

Does taking methimazole increase the chance for miscarriage?

Miscarriage is common and can occur in any pregnancy for many different reasons. One study did not find an increased chance of miscarriage in 241 women who were pregnant and used methimazole in early pregnancy. Hyperthyroidism has been associated with an increase in the chance for miscarriage.

Does taking methimazole increase the chance of birth defects?

Birth defects can happen in any pregnancy for different reasons. Out of all babies born each year, about 3 out of 100 (3%) will have a birth defect. We look at research studies to try to understand if an exposure, like methimazole, might increase the chance of birth defects in a pregnancy. Some studies and case reports suggest there could be an increased chance of birth defects when taking methimazole. There has been a suggested pattern of birth defects linked to methimazole exposure. The most commonly reported findings include aplasia cutis (ulcers on the scalp), choanal atresia (narrowing in the opening to the nasal passages), and esophageal atresia (tube connecting mouth to stomach is not formed properly). There are also studies and case reports that did not find an increase in the chance of birth defects when methimazole is taken in pregnancy. Some studies suggest that the underlying medical condition (hyperthyroidism), may play a role in the chance for birth defects. In summary, there is not enough evidence to suggest that taking methimazole clearly increases the chance of birth defects above the background chance.

Does taking methimazole in pregnancy increase the chance of other pregnancy-related problems?

One study found a higher chance of preterm delivery (birth before week 37) and low birth weight (weighing less than 5 pounds, 8 ounces [2500 grams] at birth) when methimazole was used during pregnancy. Hyperthyroidism has also been found to increase the chance for preterm delivery, smaller size (small for gestational age), and low birth weight.

Taking medications to lower thyroid levels, like methimazole, or having Graves’ disease in pregnancy can lead to thyroid levels that are either too low or too high in the fetus. If you take methimazole or if you have Graves’ disease, let your baby’s healthcare providers know, so that they can check your baby’s thyroid level after delivery.

The FDA has reported that methimazole can cause liver damage and or serious life-threatening decreases in white blood cells in people who take this medication, including women who are pregnant. There is limited information on whether methimazole use during pregnancy can cause liver damage or a decrease in white blood cells in the fetus.

Does taking methimazole in pregnancy affect future behavior or learning for the child?

Three studies looking at 56 children (from preschool to adult ages) exposed to methimazole during pregnancy found no difference in intelligence scores compared to their unexposed brothers or sisters or other unexposed people. Untreated thyroid disorders in pregnancy can increase the chance of learning problems in children.

Breastfeeding while taking methimazole:

Methimazole gets into breast milk. In 3 case series including 56 infants, and 1 study including 51 infants, methimazole in doses up to 20mg per day did not affect the breastfed infants’ thyroid function or intellectual development. Be sure to talk to your healthcare provider about all your breastfeeding questions.

If a man takes methimazole, could it affect fertility or increase the chance of birth defects?

Studies have not been done to see if methimazole could affect a man’s fertility (ability to get a woman pregnant) or increase the chance of birth defects above the background risk. In general, exposures that men have are unlikely to increase risks to a pregnancy. For more information, please see the MotherToBaby fact sheet Paternal Exposures at https://mothertobaby.org/fact-sheets/paternal-exposures-pregnancy/.

Selected References:

• Agrawal M, et al. 2022. Antithyroid drug therapy in pregnancy and risk of congenital anomalies: Systematic review and meta-analysis. Clinical endocrinology, 96(6):857–868. [PubMed: 34845757]
• Akmal A, Kung J. 2014. Propylthiouracil, and methimazole, and carbimazole-related hepatotoxicity. Expert Opin Drug Saf, 13(10):1397-1406. [PubMed: 25156887]
• Alexander EK, et al. 2017. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid: official journal of the American Thyroid Association, 27(3):315–389. [PubMed: 28056690]
• American College of Obstetricians and Gynecology. 2020. Thyroid disease in pregnancy. ACOG Practice Bulletin, Number 223. Obstet Gynecol, 135(6):e261-e274. [PubMed: 32443080]
• Andersen SL, et al. 2013. Birth Defects after early pregnancy use of antithyroid drugs: A Danish nationwide study. J Clin Endocrinol Metab, 98(11):4374-4381. [PubMed: 24151287]
• Andersen SL, et al. 2017. Birth Defects after early pregnancy use of antithyroid drugs: A Swedish nationwide study. Euro J Endocrinol, 177(4):369-378. [PubMed: 28780518]
• Andersen SL, Andersen S. 2020. Antithyroid drugs and birth defects. Thyroid Res, 13:11. [PMC free article: PMC7320591] [PubMed: 32607131]
• Arata N, et al. 2012. Pregnancy outcomes of exposure to methimazole (POEM) study: an interim report. Japanese journal of clinical medicine, 70(11):1976-1982. [PubMed: 23214071]
• Awosika AO, et al. 2023. Methimazole. StatPearls [Internet], Treasure Island (FL), available from: https://www​.ncbi.nlm​.nih.gov/books/NBK545223/
• Azizi F. 2003. Thyroid function in breast-fed infants is not affected by methimazole-induced maternal hypothyroidism: results of a retrospective study. J Endocrinol Invest, 26:301-304. [PubMed: 12841536]
• Baid SK, Merke DP. 2007. Aplasia cutis congenita following in utero methimazole exposure. J Pediatr Endocrinol Metab, 20:585-586. [PubMed: 17642418]
• Barbero P, et al. 2008. Choanal atresia associated with maternal hyperthyroidism treated with methimazole: a case-control study. Am J Med Genet, 146A(18):2390-2395. [PubMed: 18698631]
• Cassina M, et al. 2012. Pharmacologic treatment of hyperthyroidism during pregnancy. Birth Defects Res A Clin Mol Teratol, 94(8):612-619. [PubMed: 22511519]
• Chan GW, Mandel SJ. 2007. Therapy insight: management of Graves’ disease during pregnancy. Nat Clin Pract Endocrinol Metab; 3(6):470-8. [PubMed: 17515891]
• Chen CH, et al. 2011. Risk of adverse perinatal outcomes with antithyroid treatment during pregnancy: a nationwide population-based study. BJOG, 118:1365-1373. [PubMed: 21624036]
• Clementi M, et al. 1999. Methimazole embryopathy: delineation of the phenotype. Am J Med Genet, 83:43-46. [PubMed: 10076883]
• Committee on Drugs, American Academy of Pediatrics. 2001. The transfer of drugs and other chemicals into human breast milk. Pediatrics, 108:776-789.
• Cooper DS. 1987. Antithyroid drugs: to breast-feed or not to breast-feed. Am J Obstet Gynecol, 157:234-235. [PubMed: 2441601]
• Diav-Citrin O, Ornoy A. 2002. Teratogen update: antithyroid drugs-methimazole, carbimazole, and propylthiouracil. Teratology, 65:38-44. [PubMed: 11835230]
• Di Gianantonio E, et al. 2001. Adverse effects of prenatal methimazole exposure. Teratology, 64(5):262–266. [PubMed: 11745832]
• Dumi M, et al. 2002. Choanal stenosis, hypothelia, deafness, recurrent dacryocystitis, neck fistulas, short stature, and microcephaly: report of a case. Am J Med Genet, 113:295-297. [PubMed: 12439900]
• Dumitrascu MC, et al. 2021. Hyperthyroidism management during pregnancy and lactation (Review). Exp Ther Med, 22(3):960. [PMC free article: PMC8290437] [PubMed: 34335902]
• Eisenstein Z, et al. 1992. Intellectual capacity of subjects exposed to methimazole or propylthiouracil in utero. Eur J Pediatr, 151:558-559. [PubMed: 1505570]
• Ferraris S, et al. 2003. Malformations following methimazole exposure in utero: an open issue. Birth Defects Res (Part A), 67:989-992. [PubMed: 14745920]
• Francis T, et al. 2020. Safety of antithyroid drugs in pregnancy: update and therapy implications. Expert Opin Drug Saf, 19(5):565-576. [PubMed: 32223355]
• Gianetti E, et al. 2015. Pregnancy outcome in women treated with methimazole or propylthiouracil during pregnancy. J Endocrinol Invest, 38(9):977-985. [PubMed: 25840794]
• Greenberg F. 1987. Brief clinical report: Choanal atresia and athelia: Methimazole teratogenicity or a new syndrome? Am J Med Genet, 28:931-934. [PubMed: 3688031]
• Gripp KW, et al. 2011. Grade 1 microtia, wide anterior fontanel and novel type trachea-esophageal fistula in methimazole embryopathy. Am J Med Genet A, 155A(3):526-533. [PubMed: 21344626]
• Howley MM, et al. 2017. Thyroid medication use and birth defects in the National Birth Defects Prevention Study. Birth Defects Res, 109(18):1471-1481. [PMC free article: PMC10246666] [PubMed: 28758357]
• Hudzik B, Zubelewicz-Szkodzinska B. 2016. Antithyroid drugs during breastfeeding. Clin Endocrinol (Oxf), 85(6):827-830. [PubMed: 27561657]
• Johnsson E, et al. 1997. Severe malformations in infant born to hyperthyroid woman on methimazole. Lancet, 350:1520. [PubMed: 9388405]
• Karlsson FA, et al. 2002. Severe embryopathy and exposure to methimazole in early pregnancy. J Clin Endocrinol Metab, 87(2):947-949. [PubMed: 11836348]
• Korelitz JJ, et al. 2013. Prevalence of thyrotoxicosis, antithyroid medication use, and complications among pregnant women in the United States. Thyroid, 23:758-765. [PMC free article: PMC3675839] [PubMed: 23194469]
• Martin-Denavit T, et al. 2000. Ectodermal abnormalities associated with methimazole intrauterine exposure [letter]. Am J Med Genet, 94:338-340. [PubMed: 11038449]
• Methimazole tablets, USP Prescribing Information. Par Pharmaceutical Companies, Inc. Spring Valley, NY 10977. Accessed May 2024. https://www​.accessdata​.fda.gov/drugsatfda_docs​/label/2012/040350s016lbl.pdf
• Mitsuda N, et al. 1992. Risk factors for developmental disorders in infants born to women with Graves disease. Obstet Gynecol; 80(3 Pt 1):359-64. [PubMed: 1379702]
• Momotani M, et al. 1984. Maternal hyperthyroidism and congenital malformation in the offspring. Clin Endocrinol (Oxford), 20:695-700. [PubMed: 6467634]
• Purnamasari D, et al. 2019. Gastroschisis Following Treatment with High-Dose Methimazole in Pregnancy: A Case Report. Drug Saf Case Rep, 6(1):5. [PMC free article: PMC6533325] [PubMed: 31123878]
• Radetti G, et al. 2002. Foetal and neonatal thyroid disorders. Minerva Pediatr; 54(5):383-400. [PubMed: 12244277]
• Rasmussen SA, et al. 2007. Maternal thyroid disease as a risk factor for craniosynostosis. Obstetrics and gynecology, 110(2 Pt 1):369–377. [PubMed: 17666613]
• Romeo AF, Obican SG. 2020. Teratogen Update: Antithyroid Medications. Birth Defects Res, 112(15):1150-1170. [PubMed: 32738035]
• Sargent KA, et al. 1994. Apparent scalp-ear-nipple (Findlay) syndrome in a neonate exposed to methimazole in-utero. Am J Hum Genet, 55:A312.
• Sato Y, et al. 2014. A case of fetal hyperthyroidism treated with maternal administration of methimazole. J Perinatol, 34(12):945-347. [PubMed: 25421129]
• Seo GH, et al. 2018. Antithyroid drugs and congenital malformations. A nationwide Korean cohort study. Ann Intern Med, 168(6):405-413. [PubMed: 29357398]
• Seoud M, et al. 2003. Gastrointestinal malformations in two infants born to women with hyperthyroidism untreated in the first trimester. Am J Perinatol, 20:59-62. [PubMed: 12660909]
• Shepard TH, et al. 2002. Update on new developments in the study of human teratogens. Teratology, 65(4):153–161. [PubMed: 11948561]
• Stagnaro-Green A, et al. 2011. Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and postpartum. Thyroid, 21:1081-1125. [PMC free article: PMC3472679] [PubMed: 21787128]
• Tegler L, Lindstrom B. 1980. Antithyroid drugs in milk. Lancet, 2:591. [PubMed: 6158636]
• The WHO Working Group, Bennet PN (ed). 1988. Drugs and Human Lactation. Elsevier, Amsterdam, New York, Oxford, pp. 196-197.
• Tonacchera M, et al. 2020. Treatment of Graves’ hyperthyroidism with thionamides: a position paper on indications and safety in pregnancy. J Endocrinol Invest, 43(2):257-265. [PubMed: 31784912]
• Wilson LC, et al. 1998. Choanal atresia and hypothelia following methimazole exposure in utero: a second report. Am J Med Genet, 75:220-222. [PubMed: 9450891]