Propylthiouracil
MedicationPropycil
Antithyroid medication
Research strength: 3 / 5
MotherToBaby describes mixed birth-defect findings and no confirmed pattern linked to PTU alone; EMA's 2025 review considered a causal link to congenital anomalies at least a reasonable possibility, but said the overall benefit-risk balance was unchanged subject to product-information changes. A 2018 Korean claims cohort estimated 8.81 additional malformations per 1,000 live births with PTU-only prescriptions compared with no antithyroid-drug prescriptions, while a 2021 Japanese cohort found similar observed rates among women with Graves' disease treated with PTU or no medicine. The FDA's boxed warning states, "Severe liver injury and acute liver failure, in some cases fatal, have been reported in patients treated with propylthiouracil." The same label says, "Propylthiouracil should be reserved for patients who cannot tolerate methimazole and in whom radioactive iodine therapy or surgery are not appropriate treatment options." MotherToBaby and ATA identify PTU as a possible preferred option in the first trimester; ATA says evidence is insufficient to recommend PTU over methimazole after week 16, while FDA labeling says an alternative may be advisable after the first trimester. Ask your provider about your situation.
Last reviewed: 2026-10-16
Sources
- MotherToBaby: Propylthiouracil (PTU)Teratology service
- DailyMed: Propylthiouracil tablet labelDrug company label
- EMA: Propylthiouracil PSUSA scientific conclusions and product-information amendmentsEU agencyEuropean guidance
- American Thyroid Association 2026 Guidelines for Thyroid Disease in Preconception, Pregnancy, and PostpartumUS government
- Seo et al., 2018 Korean nationwide cohortUS government
- Yoshihara et al., 2021 first-trimester PTU cohortUS government
Research strength: 3 / 5
EMA's 2025 product information says studies on congenital malformations have conflicting findings and that the risk appears comparable in magnitude to untreated overt hyperthyroidism; it recommends close monitoring of the mother, fetus, and newborn when PTU is used. The FDA's boxed warning states, "Severe liver injury and acute liver failure, in some cases fatal, have been reported in patients treated with propylthiouracil." The same label says, "Propylthiouracil should be reserved for patients who cannot tolerate methimazole and in whom radioactive iodine therapy or surgery are not appropriate treatment options." FDA labeling says PTU crosses the placenta and can cause fetal goiter and cretinism; a 2009 controlled cohort reported fetal or newborn hypothyroidism and hyperthyroidism among PTU-exposed pregnancies, with most newborn thyroid tests normalizing during the first month. MotherToBaby says PTU or Graves' disease can lead to low or high thyroid levels in a baby and recommends a thyroid check after delivery; FDA says an alternative may be advisable after the first trimester, while ATA says evidence is insufficient to recommend PTU over methimazole after week 16. Ask your provider about your situation.
Last reviewed: 2026-10-16
Sources
- EMA: Propylthiouracil PSUSA scientific conclusions and product-information amendmentsEU agencyEuropean guidance
- DailyMed: Propylthiouracil tablet labelDrug company label
- MotherToBaby: Propylthiouracil (PTU)Teratology service
- Rosenfeld et al., 2009 controlled cohortUS government
- American Thyroid Association 2017 pregnancy guidelineUS government
- American Thyroid Association 2026 Guidelines for Thyroid Disease in Preconception, Pregnancy, and PostpartumUS government
Research strength: 3 / 5
EMA's 2025 product information says birth-defect studies have conflicting findings, that risk appears comparable in magnitude to untreated overt hyperthyroidism, and that close maternal, fetal, and newborn monitoring is recommended when PTU is used. The FDA's boxed warning states, "Severe liver injury and acute liver failure, in some cases fatal, have been reported in patients treated with propylthiouracil." The same label says, "Propylthiouracil should be reserved for patients who cannot tolerate methimazole and in whom radioactive iodine therapy or surgery are not appropriate treatment options." In a 2009 controlled cohort, 9.5% of PTU-exposed fetuses or newborns had hypothyroidism and 10.3% had hyperthyroidism, which the authors said could be related to maternal disease; most newborn thyroid tests normalized within the first month. MotherToBaby says PTU or Graves' disease can lead to low or high thyroid levels in a baby and recommends checking the baby's thyroid after delivery. FDA labeling says an alternative may be advisable after the first trimester, while ATA says evidence is insufficient to recommend PTU over methimazole after week 16. Ask your provider about your situation.
Last reviewed: 2026-10-16
Sources
- EMA: Propylthiouracil PSUSA scientific conclusions and product-information amendmentsEU agencyEuropean guidance
- DailyMed: Propylthiouracil tablet labelDrug company label
- Rosenfeld et al., 2009 controlled cohortUS government
- MotherToBaby: Propylthiouracil (PTU)Teratology service
- American Thyroid Association 2026 Guidelines for Thyroid Disease in Preconception, Pregnancy, and PostpartumUS government
Propylthiouracil — Breastfeeding
Research strength: 2 / 5
The FDA's boxed warning states, "Severe liver injury and acute liver failure, in some cases fatal, have been reported in patients treated with propylthiouracil." The same label says, "Propylthiouracil should be reserved for patients who cannot tolerate methimazole and in whom radioactive iodine therapy or surgery are not appropriate treatment options." MotherToBaby and LactMed report that PTU enters breastmilk in small amounts; a study of nine nursing women found that 0.025% of a 400 mg dose was excreted in milk over four hours. LactMed reports no known cases of PTU-related liver damage in breastfed infants but says it is unknown whether small amounts in milk can cause liver damage; its small follow-up studies include some infant thyroid-test changes that later resolved. FDA labeling says milk exposure likely gives a clinically insignificant infant dose, while LactMed notes some experts prefer methimazole because liver injury is more common with PTU and says doses should be limited to 750 mg daily, preferably 450 mg. Ask your provider about your situation.
Last reviewed: 2026-10-16
Pregnancy
Evidence is observational and the comparison groups differ. In the Korean nationwide claims cohort (Seo et al., 2018), 2,886,970 completed pregnancies were linked to live-born infants; first-trimester PTU-only prescriptions were associated with 8.81 additional malformations per 1,000 live births (95% CI 3.92 to 13.70) versus pregnancies without antithyroid-drug prescriptions. Prescription claims do not confirm ingestion, the comparator may include untreated thyroid disease, and the study included live births. In the Japanese 2012 Graves' cohort (6,744 known pregnancy outcomes, 5,967 live births), major anomalies were 1.9% (21/1,399) with PTU and 2.1% (40/1,906) among women given no first-trimester medication (P=0.709). In a later Japanese cohort, anomalies were 5.5% (30/541) with PTU and 5.7% (27/475) among untreated women with Graves' disease. In the Israeli prospective cohort (Rosenfeld et al., 2009), 115 PTU-exposed pregnancies and 1,141 controls were followed; major anomalies were 1/80 (1.3%) versus 34/1,066 (3.2%; P=0.507), hypothyroidism occurred in 9.5% of fetuses or newborns, and hyperthyroidism in 10.3%, possibly related to maternal disease. Most neonatal thyroid tests normalized during the first month. EMA's 2025 product information describes conflicting epidemiological findings, a slight increase in some comparisons with women without hyperthyroidism, and a risk comparable in magnitude to untreated overt hyperthyroidism; it recommends individual benefit-risk assessment and close maternal, fetal, and neonatal monitoring. The 2026 ATA guideline prefers PTU during the first trimester because associated defects are generally less severe than with methimazole, and says evidence is insufficient to recommend PTU over methimazole after 16 weeks. A 2026 Danish study described treatment practice in 121 women rather than comparing child outcomes. A 2026 neonatal metabolomics study compared 31 PTU-exposed neonates with 36 healthy controls and reported lower histidine and aspartic acid, but no clinical outcomes or hyperthyroid untreated comparison; it does not establish a clinical effect caused by PTU.
Breastfeeding
Kampmann et al. (1980) measured PTU in blood and milk from nine lactating women after a 400 mg dose. Mean milk concentration was 0.7 micrograms/mL at 1.5 hours; 99 micrograms, or 0.025% of the dose, was excreted over four hours. One infant was followed for five months while the mother took 200 to 300 mg daily, with no change in thyroid parameters. LactMed's 15 June 2026 review also summarizes small infant follow-up reports: mild early thyroid-test changes generally normalized, some infants had prenatal PTU exposure, and no infant levels were available. LactMed reports no cases of PTU-induced liver damage in breastfed infants but says whether small milk amounts can cause liver injury is unknown. The evidence base is small and does not establish the absence of rare infant effects.
MotherToBaby describes propylthiouracil (PTU) as a medicine used for hyperthyroidism and Graves' disease that lowers the amount of thyroid hormone made by the thyroid gland, and identifies Propycil as one brand name. The consulted DailyMed source is a generic PTU tablet label, not a Propycil-specific label; this draft does not generalize product-specific inactive ingredients.
What was tested
Pregnancy studies include nationwide prescription-claims cohorts, single-center retrospective cohorts, and a prospective teratology-service cohort. They measure prescriptions or reported exposure and are not randomized treatment comparisons; disease severity, maternal thyroid levels, exposure timing, and selection of live births can affect comparisons. Breastfeeding evidence includes small pharmacokinetic milk-level studies and infant case follow-up; LactMed found no published infant serum-level data as of its June 2026 revision. The 2026 metabolomics study is exploratory and uses healthy controls, so it cannot separate drug exposure from maternal hyperthyroidism.
How to describe the chance of congenital anomalies with PTU in pregnancy
- Source A says
- EMA's 2025 PSUSA considers a causal relationship between PTU use in pregnancy and congenital anomalies at least a reasonable possibility and required product-information changes. The amended information says epidemiological studies conflict and that risk appears comparable in magnitude to untreated overt hyperthyroidism. (EMA: Propylthiouracil PSUSA scientific conclusions and product-information amendments)
- Source B says
- MotherToBaby reports mixed findings with no confirmed pattern linked to PTU alone. ATA 2026 prefers PTU in the first trimester because associated defects are generally less severe than those associated with methimazole. (MotherToBaby: Propylthiouracil (PTU), American Thyroid Association 2026 Guidelines for Thyroid Disease in Preconception, Pregnancy, and Postpartum)
- Why they differ
- The sources use different evidence thresholds and comparators. Observational cohorts compare PTU prescriptions with no antithyroid-drug prescriptions, untreated Graves' disease, or nonteratogen controls; disease severity and treatment selection can confound these comparisons.
- What the evidence shows overall
- The 2025 EMA action should be disclosed, but it does not say PTU must be stopped: EMA also says the benefit-risk balance is unchanged subject to product-information changes. The Korean cohort found a small absolute difference versus no antithyroid-drug prescriptions, while Japanese cohorts found similar observed rates in their selected untreated comparison groups. Current ATA guidance still favors PTU in the first trimester and does not establish a preferred drug after week 16.
Whether PTU should be changed after the first trimester
- Source A says
- FDA labeling says an alternative antithyroid medication may be advisable after the first trimester because of potential maternal liver injury. (DailyMed: Propylthiouracil tablet, USP label)
- Source B says
- ATA 2017 says it remained unclear whether to continue PTU or change to methimazole after 16 weeks and made no recommendation; ATA 2026 says evidence is insufficient to recommend PTU over methimazole after 16 weeks. (American Thyroid Association 2017 pregnancy guideline, American Thyroid Association 2026 Guidelines for Thyroid Disease in Preconception, Pregnancy, and Postpartum)
- Why they differ
- The label gives a caution about PTU's maternal liver risk, while ATA weighs that concern against fetal medication exposure and the limited evidence on switching and disease control.
- What the evidence shows overall
- The label's wording is advisory, not an automatic switch instruction. The current guideline does not identify a superior drug after 16 weeks; the summaries preserve both positions and the need for clinician-guided management.
PTU during breastfeeding and possible infant liver effects
- Source A says
- FDA labeling says the small amount of PTU in breastmilk likely gives the nursing infant a clinically insignificant dose. (DailyMed: Propylthiouracil tablet, USP label)
- Source B says
- LactMed reports no known cases of PTU-induced liver damage in breastfed infants but says it is unknown whether small amounts in milk can cause liver damage; some experts now prefer methimazole because liver injury rates are higher with PTU. (LactMed (NIH): Propylthiouracil)
- Why they differ
- The label emphasizes measured low milk transfer, while LactMed also considers PTU's maternal liver-injury profile and the lack of infant outcome data for rare liver effects.
- What the evidence shows overall
- The milk-transfer study supports low exposure, but infant follow-up is small and does not resolve rare liver outcomes. The corrected breastfeeding summary reports the low transfer and preserves LactMed's uncertainty and expert preference difference.
- EMA's 2025 product information says an individual benefit-risk assessment is needed before PTU treatment during pregnancy, recommends the lowest effective dose, and recommends close maternal, fetal, and newborn monitoring when PTU is used. (European Medicines Agency)
- FDA labeling says an alternative antithyroid medication may be advisable after the first trimester. (DailyMed)
- ATA 2026 says evidence is insufficient to recommend PTU over methimazole after 16 weeks. (American Thyroid Association)
- LactMed says PTU doses during breastfeeding should be limited to 750 mg daily, preferably 450 mg daily. (LactMed (NIH))
General research, not specific to pregnancy.
FDA labeling includes a boxed warning for severe liver injury and acute liver failure, sometimes fatal, and says PTU should be reserved for patients who cannot tolerate methimazole when radioactive iodine or surgery are not appropriate.
- The label reports agranulocytosis in approximately 0.2% to 0.5% of patients, calls it potentially life-threatening, and says it typically occurs within the first three months of therapy. (DailyMed: Propylthiouracil tablet, USP label)
- EMA's October 2021 PRAC signal table lists drug reaction with eosinophilia and systemic symptoms (DRESS) for PTU under routine pharmacovigilance, not as a product-information-change recommendation. (EMA PRAC recommendations of signals adopted 25-28 October 2021)
- A 2025 single-patient report described PTU-associated agranulocytosis and pancytopenia with severe infection; it is a case report, not an estimate of frequency. (Svenningsen and Marina, 2025 PTU-induced agranulocytosis case report)
- MotherToBaby: Propylthiouracil (PTU)Teratology serviceSecondary
- DailyMed: Propylthiouracil tablet, USP labelDrug company labelTertiaryPage dated 2026-07-01
- EMA: Propylthiouracil PSUSA scientific conclusions and product-information amendmentsEU agencySecondaryPage dated 2025-10-01Independent checkEuropean guidance
- American Thyroid Association 2026 Guidelines for Thyroid Disease in Preconception, Pregnancy, and PostpartumUS governmentSecondaryPage dated 2026-05-31Independent check
- American Thyroid Association 2017 pregnancy guidelineUS governmentSecondaryIndependent check
- Seo et al., 2018 Korean nationwide cohortUS governmentPrimaryIndependent check
- Yoshihara et al., 2012 Graves' disease pregnancy cohortUS governmentPrimaryIndependent check
- Yoshihara et al., 2021 first-trimester PTU cohortUS governmentPrimaryIndependent check
- Rosenfeld et al., 2009 controlled PTU cohortUS governmentPrimaryIndependent check
- Hayashida et al., 1990 neonatal hepatic injury case reportUS governmentPrimaryIndependent check
- LactMed (NIH): PropylthiouracilUS governmentSecondaryPage dated 2026-06-15Independent check
- Kampmann et al., 1980 human milk studyUS governmentPrimaryIndependent check
- EMA PRAC recommendations of signals adopted 25-28 October 2021EU agencySecondaryEuropean guidance
- Uldall-Torp et al., 2026 Danish PRETHYR treatment-practice studyUS governmentPrimary
- Song et al., 2026 neonatal metabolomics studyUS governmentPrimary
- Svenningsen and Marina, 2025 PTU-induced agranulocytosis case reportUS governmentPrimary
Last reviewed 2026-10-16
Independent check 2026-10-16
Not yet clinician-reviewed
Ask your provider about your situation.