Propylthiouracil (PTU) During Pregnancy: First Trimester Use

What does the evidence say about propylthiouracil during pregnancy?

1. MotherToBaby reports that FDA and ACOG consider PTU a possible preferred antithyroid medicine during the first trimester when medication is needed, while emphasizing that poorly controlled hyperthyroidism can cause thyroid storm, heart failure, pregnancy loss, preterm birth, and other serious complications. 1
2. The FDA label warns that PTU has caused severe liver injury, liver failure, transplantation, and death; injury is unpredictable, routine biochemical monitoring is not expected to eliminate risk, and symptoms such as poor appetite, itching, right-upper-abdominal pain, dark urine, or jaundice require immediate prescriber contact. 2
3. LiverTox estimates clinically apparent PTU liver injury at at least 1 in 1,000 exposed people and acute liver failure at about 1 in 10,000 adults, usually beginning within 2 to 12 weeks but sometimes later; hyperthyroidism itself can also alter liver tests and complicate attribution. 3
4. A 2023 meta-analysis of 13 observational studies found a lower overall birth-defect association with PTU than methimazole, while differences in miscarriage and liver toxicity were not statistically significant; these nonsignificant results do not prove equal risk, and disease severity and exposure definitions varied across studies. 4
5. A Danish nationwide live-birth cohort included 564 early PTU exposures and found an adjusted birth-defect odds ratio of 1.41 compared with pregnancies without antithyroid-drug prescriptions; pharmacy dispensing, live-birth restriction, and absence of a clean disease-matched control limit causal interpretation. 5

Is propylthiouracil safe in each trimester?

• First trimester. PTU is often preferred over methimazole during the first trimester because methimazole has been linked to a more recognizable pattern of uncommon birth defects. PTU is not risk-free, and a large Danish live-birth cohort also found an association between early PTU exposure and defects. Disease severity and prescription-record limitations make causation uncertain.
• Second trimester. After the first trimester, the prescriber may consider switching from PTU to methimazole because severe PTU liver injury can occur unpredictably. A switch is not automatic and should not be self-directed. Thyroid tests guide the lowest effective amount because both the medicine and poorly controlled hyperthyroidism can affect pregnancy.
• Third trimester. Continue scheduled thyroid monitoring and tell the newborn team about antithyroid medicine exposure. PTU crosses the placenta and can cause fetal or newborn low thyroid function or goiter if exposure is excessive, while maternal Graves antibodies can also affect fetal thyroid status. Delivery and newborn plans should reflect both the disease and the medicine.