Antithyroid Drugs in Pregnancy
A 32-year-old woman at 10 weeks gestation is diagnosed with Graves' disease. Her physician is choosing between carbimazole (CMZ) and propylthiouracil (PTU). Which of the following statements best justifies preferring PTU during the first trimester?
Correct Answer — B
Carbimazole has been associated with fetal aplasia cutis — a scalp skin defect — and other rare teratogenic malformations, particularly in the first trimester (the critical organogenesis window). PTU is therefore preferred in the first trimester. After the first trimester, the risk calculus shifts because PTU carries significant hepatotoxicity risk; some guidelines switch back to CMZ after 12–14 weeks. PTU is also excreted less in breast milk, making it preferable for breastfeeding.
Key concept
First trimester of pregnancy: PTU preferred over CMZ due to CMZ-associated fetal aplasia cutis. PTU also less excreted in breast milk.