Propylthiouracil Use in Hyperthyroidism: Indications, Pregnancy Management, and Safety

Indications and Emergency Use

The American College of Endocrinology recommends propylthiouracil (PTU) for first‑trimester pregnancy and for thyroid storm as part of multimodal emergency management, because methimazole is linked to specific congenital malformations and PTU is effective in acute decompensation. [@1]
PTU is indicated when methimazole is not tolerated and definitive therapy (radioactive iodine or surgery) is unsuitable, serving as a bridge to definitive treatment. [@1]

Upon a positive pregnancy test, patients on methimazole should be switched immediately to PTU to avoid first‑trimester teratogenicity (e.g., aplasia cutis, choanal atresia, esophageal atresia). [@2]
At the beginning of the second trimester, therapy should be transitioned back to methimazole to reduce the higher risk of severe, potentially fatal liver injury associated with PTU. [@2]
Target free T4 at or slightly above the upper limit of the non‑pregnant reference range (or total T4 ≈ 1.5 × upper limit) while keeping TSH suppressed to maintain euthyroidism in pregnancy. [@2]
Thyroid function tests should be monitored monthly throughout pregnancy, with dose adjustments made as needed to maintain target hormone levels. [@2]
TSH‑receptor antibodies should be measured at 22–26 weeks gestation to assess the risk of fetal or neonatal thyrotoxicosis. [@2]

PTU should not be used as first‑line therapy for Graves’ disease outside the three specific scenarios (first‑trimester pregnancy, thyroid storm, methimazole intolerance); methimazole remains the preferred agent for the majority of patients. [@1]
PTU carries a black‑box warning for severe hepatotoxicity, including fatal hepatic necrosis and cases requiring liver transplantation (evidence from regulatory safety data). (no citation required per instruction)

PTU requires three‑times‑daily dosing (approximately every 8 hours), which may reduce adherence compared with once‑daily methimazole. [@5]
The dose conversion ratio from methimazole to PTU is roughly 20–30 : 1 (e.g., 300 mg PTU ≈ 10–15 mg methimazole). [@2]
When PTU is continued beyond the first trimester, liver function tests should be checked every 4 weeks; prospective data have not demonstrated that this monitoring prevents fulminant hepatotoxicity. [@2]

Do not use PTU as routine first‑line therapy; reserve it exclusively for the three FDA‑approved scenarios to minimize hepatotoxicity risk. [@1]
Do not delay switching to methimazole in the second trimester when full‑dose antithyroid therapy is required; the risk of PTU‑related liver injury outweighs the teratogenic risk of methimazole after organogenesis. [@2]
Do not substitute PTU solely to avoid agranulocytosis, as PTU carries a comparable or higher risk of agranulocytosis plus a substantially higher hepatotoxicity risk compared with methimazole. [@5]
Do not use PTU during breastfeeding; methimazole is preferred because PTU poses a higher risk of hepatic necrosis to the mother or infant. [@2]