Methimazole
MedicationTapazole
Antithyroid medication
Research strength: 4 / 5
MotherToBaby says the evidence is not enough to show that methimazole clearly increases the chance of birth defects above background. Danish and Korean register studies by Andersen (2013, 2019) and Seo (2018) found that early-pregnancy methimazole or carbimazole exposure was linked with more birth defects, but they cannot prove the medicine caused them. In Di Gianantonio's 2001 prospective study of 241 exposed pregnancies, researchers found no increase in overall major birth defects or pregnancy loss, though two infants had defects in the reported pattern. The ATA's 2017 summary says PTU had lower birth-defect risk and was the recommended treatment during the first trimester; it did not recommend a routine switch back after that because doses are not equivalent. The MHRA's 2025 notice advises women who can become pregnant to use effective contraception during treatment and says pregnancy use needs a strict individual benefit-risk assessment, the lowest effective dose, no added thyroid hormone, and close maternal, fetal, and newborn monitoring. Ask your provider about your situation.
Last reviewed: 2026-10-16
Sources
- MotherToBaby: MethimazoleTeratology service
- American Thyroid Association: 2017 guideline summary
- American Thyroid Association: Hyperthyroidism in Pregnancy
- Di Gianantonio et al., Teratology 2001US government
- Andersen et al., JCEM 2013US government
- Seo et al., Annals of Internal Medicine 2018US government
- Andersen et al., JCEM 2019US government
- MHRA: Carbimazole or thiamazole safety communication, listed in August 2025 roundupUK government
Research strength: 3 / 5
DailyMed says methimazole crosses the placenta and can enlarge the baby's thyroid or lower its function. The label says that switching from PTU to methimazole may be preferable in the second and third trimesters because PTU can injure the parent's liver. The current ATA pregnancy page instead says PTU is preferred until week 16, while its 2017 summary did not recommend a routine switch back after the first trimester. The MHRA advises women who can become pregnant to use effective contraception during treatment and says pregnancy use needs a strict individual benefit-risk assessment, the lowest effective dose, no added thyroid hormone, and close monitoring of the mother, fetus, and newborn. Ask your provider about your situation.
Last reviewed: 2026-10-16
Sources
- DailyMed: Methimazole Tablets, USP labelDrug company label
- American Thyroid Association: Hyperthyroidism in Pregnancy
- American Thyroid Association: 2017 guideline summary
- MHRA: Carbimazole or thiamazole safety communication, listed in August 2025 roundupUK government
Research strength: 3 / 5
DailyMed says thyroid activity may lessen as pregnancy advances, so the dose may be lowered or treatment stopped weeks or months before delivery in some cases. The label says switching from PTU to methimazole may be preferable in the second and third trimesters because PTU can injure the parent's liver. The current ATA pregnancy page prefers PTU until week 16, while its 2017 summary did not recommend a routine switch back after the first trimester. The MHRA advises women who can become pregnant to use effective contraception during treatment and says methimazole use during pregnancy needs an individual benefit-risk assessment, the lowest effective dose, no added thyroid hormone, and close maternal, fetal, and newborn monitoring. Ask your provider about your situation.
Last reviewed: 2026-10-16
Sources
- DailyMed: Methimazole Tablets, USP labelDrug company label
- American Thyroid Association: Hyperthyroidism in Pregnancy
- American Thyroid Association: 2017 guideline summary
- MHRA: Carbimazole or thiamazole safety communication, listed in August 2025 roundupUK government
Methimazole — Breastfeeding
Research strength: 4 / 5
LactMed reports that doses up to 20 mg per day did not affect breastfed infants' thyroid function or intellectual development, and says some experts consider methimazole the antithyroid drug of choice during nursing. A 2000 study of 139 mothers and their infants found infant thyroid tests stayed normal during a year of maternal treatment. The 2017 ATA summary supports doses up to 20 mg per day with routine growth and development checks, while the current ATA page says infant thyroid testing is not required. DailyMed says methimazole enters breast milk, but its request for thyroid checks every week or two does not say whether the mother or infant is to be tested. Ask your provider about your situation.
Last reviewed: 2026-10-16
Sources
- LactMed: MethimazoleUS government
- Azizi et al., JCEM 2000US government
- American Thyroid Association: 2017 guideline summary
- American Thyroid Association: Hyperthyroidism in Pregnancy
- DailyMed: Methimazole Tablets, USP labelDrug company label
Pregnancy
Di Gianantonio et al. (2001) prospectively followed 241 methimazole-exposed pregnancies and 1,089 comparison pregnancies referred to European teratology services. The study found no increase in the general rate of major anomalies or pregnancy loss, but two newborns had malformations in the proposed methimazole embryopathy pattern. Andersen et al. (2013) studied 817,093 live births in Denmark; early MMI/carbimazole exposure was associated with more birth defects, and the MMI/carbimazole group had a different malformation pattern from the PTU group. Seo et al. (2018) analyzed 2,886,970 completed pregnancies in Korea; first-trimester antithyroid prescriptions, especially MMI alone or combined with PTU, were associated with more malformations, and the study reported a dose association for MMI. Andersen et al. (2019) extended the Danish register cohort to 1,243,353 children and reported a higher birth-defect frequency after early MMI/carbimazole exposure than among nonexposed children. These observational studies cannot prove the medicine caused the outcomes, and prescriptions do not establish the exact dose taken. Chen et al. (2011) found higher low-birth-weight odds with PTU, while its abstract reported no increased adverse fetal outcomes in the MMI group versus untreated hyperthyroid mothers. The MotherToBaby statement that one study linked MMI to preterm birth and low birth weight could not be traced to a named primary study in the saved fact sheet and is excluded from the public summaries. The 2026 ATA guideline replaces the 2017 guideline; its available abstract confirms updated recommendations but does not reproduce the specific antithyroid-drug recommendation. The 2026 Danish PRETHYR report described treatment patterns among 121 women, not comparative safety outcomes. A 2026 Taiwanese cohort examined maternal thyroid disease and medications including MMI, but its abstract reports no MMI-specific neurodevelopment estimate.
Breastfeeding
LactMed's record, revised June 15, 2026, summarizes small milk-level studies and observational infant follow-up. Azizi et al. (2000) studied 139 thyrotoxic mothers and their infants; infant thyroid measures remained normal during follow-up through 12 months of maternal treatment, and the study also assessed child development. Azizi et al. (2003) compared 42 breastfed children exposed to maternal MMI with 40 controls at ages 48 to 86 months and found no differences in thyroid tests, growth, or measured intellectual development. LactMed reports no cases of altered thyroid function among infants exposed through breast milk and says evidence includes doses up to 20 mg daily. These were not randomized trials, and the sample sizes do not establish that every dose or individual circumstance has the same outcome.
The reviewed DailyMed generic tablet label lists methimazole as the active ingredient and lactose monohydrate, magnesium stearate, corn starch, and talc as inactive ingredients. Ingredients may differ across products. DailyMed's current product listings included no Tapazole-specific label in the search.
What was tested
Pregnancy evidence includes prospective teratology-service follow-up and large registry or insurance-claims cohorts. Most studies are observational, so they identify associations rather than prove cause. Breastfeeding evidence includes milk and infant drug-level studies and follow-up of infant thyroid function, growth, and development. No randomized trial testing methimazole exposure during pregnancy was identified in the reviewed sources.
Whether early-pregnancy methimazole clearly increases birth-defect risk
- Source A says
- MotherToBaby says the evidence is not enough to show a clear increase above background. The prospective 2001 teratology-service study found no increase in the overall rate of major anomalies or pregnancy loss, although two newborns had defects in the proposed pattern. (MotherToBaby: Methimazole, Di Gianantonio et al., Teratology 2001)
- Source B says
- Large registry studies from Denmark and Korea found associations between early MMI or carbimazole exposure and birth defects. The 2017 ATA summary says PTU generally has a lower birth-defect risk and is recommended in the first trimester, and the MHRA strengthened its pregnancy warning in 2025. (Andersen et al., JCEM 2013, Danish nationwide study, Seo et al., Annals of Internal Medicine 2018, Korean nationwide cohort, Andersen et al., JCEM 2019, Danish nationwide cohort, American Thyroid Association: 2017 guideline summary, MHRA: Carbimazole or thiamazole DHPC, risk of acute pancreatitis and strengthened advice on contraception)
- Why they differ
- The prospective teratology-service cohort was smaller and found no overall increase, while larger register studies had more power to detect associations and a dose pattern. The studies differ in population, exposure measurement, and ability to separate medicine effects from the underlying thyroid disease and its severity.
- What the evidence shows overall
- The larger cohorts and pregnancy guidance support an early-pregnancy signal and a characteristic defect pattern, but observational data cannot prove cause or fully separate medicine exposure from disease and treatment factors. The summary names both the signal and the uncertainty.
Whether to switch from PTU to methimazole after the first trimester
- Source A says
- The 2017 ATA summary says PTU is preferred in the first trimester and that the task force did not recommend a routine switch back after the first trimester because the drugs are not dose-equivalent. The current ATA pregnancy page says PTU is preferred until week 16. (American Thyroid Association: 2017 guideline summary, American Thyroid Association: Hyperthyroidism in Pregnancy)
- Source B says
- DailyMed says it may be preferable to switch from PTU to methimazole for the second and third trimesters because of potential maternal liver injury with PTU. (DailyMed: Methimazole Tablets, USP label, RemedyRepack Inc.)
- Why they differ
- The label weighs maternal liver injury from PTU, while the 2017 ATA task force emphasized that the medicines are not dose-equivalent and did not recommend a routine switch. The available current ATA patient page gives a week-16 preference, while the abstract of the 2026 guideline does not reproduce its specific switch recommendation.
- Other sources
- The MHRA's 2025 communication requires an individual benefit-risk assessment, the lowest effective dose without added thyroid hormone, and close maternal, fetal, and newborn monitoring, but does not specify a trimester for switching. (MHRA: Carbimazole or thiamazole DHPC, risk of acute pancreatitis and strengthened advice on contraception)
- What the evidence shows overall
- The evidence supports PTU preference in early pregnancy and recognizes maternal liver concerns with PTU later, but accessible sources do not establish a routine switch-back rule. Do not present the label's option as a consensus recommendation.
Thyroid testing and follow-up for breastfed infants
- Source A says
- DailyMed says to monitor thyroid function at frequent weekly or biweekly intervals in its nursing section, without specifying whose thyroid should be tested. The 2017 ATA summary calls for monitoring the child's growth and development during routine pediatric visits. (DailyMed: Methimazole Tablets, USP label, RemedyRepack Inc., American Thyroid Association: 2017 guideline summary)
- Source B says
- The current ATA page says thyroid-status monitoring of breastfed infants is not required, and LactMed reports no cases of altered infant thyroid function with maternal doses up to 20 mg per day. (American Thyroid Association: Hyperthyroidism in Pregnancy, LactMed: Methimazole)
- Why they differ
- The label's monitoring sentence does not name the person to be tested. Routine growth and development checks are not the same as weekly or biweekly infant thyroid blood tests. Current ATA guidance and LactMed reflect observational infant data and do not call for routine infant thyroid testing at the described doses.
- Other sources
- MotherToBaby also reports no effect on infant thyroid function or development in the studies it summarizes at doses up to 20 mg per day. (MotherToBaby: Methimazole)
- What the evidence shows overall
- Do not describe the DailyMed line as an instruction to test the infant. The public text states that the label is ambiguous and gives the current ATA and LactMed position alongside the 2017 routine growth and development guidance.
- The MHRA says women who can become pregnant should use effective contraception during treatment with thiamazole or carbimazole. If treatment is used during pregnancy, it calls for an individual benefit-risk assessment, the lowest effective dose without added thyroid hormone, and close maternal, fetal, and newborn monitoring. (MHRA)
- LactMed says taking methimazole just after nursing and waiting three to four hours before nursing again may reduce the infant dose. (LactMed)
- DailyMed says patients who become pregnant or plan to become pregnant while taking an antithyroid drug should contact their physician promptly about treatment. (DailyMed)
General research, not specific to pregnancy.
The label warns about serious blood-cell, liver, and blood-vessel reactions. The MHRA's 2025 action also warns that acute pancreatitis can return sooner after re-exposure.
- DailyMed says agranulocytosis, a severe drop in infection-fighting white blood cells, can be life-threatening and tells patients to report fever or sore throat promptly. (DailyMed: Methimazole Tablets, USP label, RemedyRepack Inc.)
- The MHRA says thiamazole or carbimazole should be stopped immediately after treatment-associated acute pancreatitis and must not be restarted because re-exposure may cause an earlier recurrence, which can be life-threatening. (MHRA: Carbimazole or thiamazole DHPC, risk of acute pancreatitis and strengthened advice on contraception)
- DailyMed reports liver injury, including acute liver failure, and cases of vasculitis with severe complications. (DailyMed: Methimazole Tablets, USP label, RemedyRepack Inc.)
- MotherToBaby: MethimazoleTeratology serviceSecondary
- DailyMed: Methimazole Tablets, USP label, RemedyRepack Inc.Drug company labelTertiaryPage dated 2026-09-10
- American Thyroid Association: 2017 guideline summarySecondaryIndependent check
- American Thyroid Association: Hyperthyroidism in PregnancySecondaryIndependent check
- Korevaar et al.: American Thyroid Association 2026 Guidelines for Thyroid Disease in Preconception, Pregnancy, and Postpartum, PubMed abstractUS governmentSecondary
- LactMed: MethimazoleUS governmentSecondaryPage dated 2026-06-15Independent check
- MHRA: Carbimazole or thiamazole DHPC, risk of acute pancreatitis and strengthened advice on contraceptionUK governmentSecondaryIndependent check
- MHRA Safety Roundup: August 2025UK governmentSecondaryPage dated 2025-08-28
- Di Gianantonio et al., Teratology 2001US governmentPrimaryIndependent check
- Andersen et al., JCEM 2013, Danish nationwide studyUS governmentPrimaryIndependent check
- Seo et al., Annals of Internal Medicine 2018, Korean nationwide cohortUS governmentPrimaryIndependent check
- Andersen et al., JCEM 2019, Danish nationwide cohortUS governmentPrimaryIndependent check
- Chen et al., BJOG 2011, nationwide population-based studyUS governmentPrimaryIndependent check
- Azizi et al., JCEM 2000, methimazole during breastfeedingUS governmentPrimaryIndependent check
- Azizi et al., Journal of Pediatric Endocrinology and Metabolism 2003US governmentPrimaryIndependent check
- Uldall-Torp et al., JCEM 2026, Danish PRETHYR studyUS governmentPrimaryIndependent check
- Wu et al., European Child & Adolescent Psychiatry 2026US governmentPrimaryIndependent check
Last reviewed 2026-10-16
Independent check 2026-10-16
Not yet clinician-reviewed
Ask your provider about your situation.