A 28-year-old woman with newly diagnosed Graves' disease is planning pregnancy in 6 months. She is euthyroid on antithyroid therapy. Which drug should be continued as first-line maintenance therapy to minimize teratogenic risk?

Explanation

## Antithyroid Drug Choice in Pregnancy **Key Point:** Propylthiouracil (PTU) is the drug of choice in pregnancy (especially first trimester) because methimazole is associated with a rare but distinct embryopathy (methimazole embryopathy) characterized by esophageal atresia, choanal atresia, and cutaneous abnormalities. ## Methimazole Embryopathy **High-Yield:** Methimazole embryopathy (also called "methimazole fetopathy") includes: - Esophageal atresia - Choanal atresia - Cutaneous abnormalities (aplasia cutis) - Occurs with first-trimester exposure ## Antithyroid Drug Comparison in Pregnancy | Feature | PTU | Methimazole | |---------|-----|-------------| | **First-trimester safety** | Safe | Teratogenic (embryopathy) | | **Placental transfer** | Minimal | Moderate | | **Agranulocytosis risk** | 0.1–0.5% | 0.01% | | **Hepatotoxicity** | Rare | Rare | | **Preferred in pregnancy** | Yes (1st trimester) | No | | **Preferred for maintenance** | No (if not pregnant) | Yes (non-pregnant) | **Clinical Pearl:** Many guidelines recommend PTU during the first trimester, then switching to methimazole in the second and third trimesters (if needed) to reduce PTU hepatotoxicity risk during prolonged exposure. ## Pregnancy Counseling Strategy 1. **Pre-conception (now):** Continue PTU until conception confirmed 2. **First trimester:** Continue PTU 3. **Second/third trimester:** May consider switch to methimazole (lower agranulocytosis risk with long-term use) 4. **Postpartum:** Can resume methimazole if non-breastfeeding; PTU safe if breastfeeding **Warning:** Do NOT use methimazole in first trimester due to embryopathy risk. Propranolol is a beta-blocker and does not treat the underlying thyroid disorder — it is an adjunct only. [cite:Harrison 21e Ch 397]