A 28-year-old woman from Mumbai, 8 weeks pregnant, presents with palpitations, anxiety, and weight loss. She has a history of Graves' disease diagnosed 2 years ago and was treated with antithyroid drugs (PTU) for 18 months, after which treatment was stopped. On examination, she has a diffuse goiter and resting heart rate of 115 bpm. TSH is <0.01 mIU/L, free T4 is 25 pg/dL, and free T3 is 7.8 pg/dL. She is concerned about the safety of treatment during pregnancy. Which is the most appropriate initial management?

Explanation

## Management of Hyperthyroidism in Pregnancy ### Clinical Context **Key Point:** Untreated maternal hyperthyroidism in pregnancy increases risks of: - Miscarriage, preterm labor, intrauterine growth restriction - Neonatal thyroid storm, tachycardia, and heart failure - Maternal thyroid storm and atrial fibrillation Therefore, treatment is **mandatory**. ### Why PTU Is the Drug of Choice in Pregnancy | Parameter | PTU | Methimazole | |-----------|-----|-------------| | **Placental transfer** | Minimal (highly protein-bound) | Moderate (crosses placenta) | | **Fetal risk** | Safe; no teratogenicity | **Methimazole embryopathy** (rare but documented: esophageal atresia, choanal atresia, aplasia cutis) | | **Mechanism** | Blocks synthesis + peripheral conversion of T4→T3 | Blocks synthesis only | | **Agranulocytosis risk** | ~0.1–0.3% (monitor CBC) | Lower (~0.01%) | | **Hepatotoxicity** | Rare but severe | Rare | | **Pregnancy recommendation** | **First trimester preferred** | Avoid in first trimester | **High-Yield:** PTU is the **gold standard** for first-trimester hyperthyroidism because it has minimal placental transfer and no known teratogenic effects. Methimazole carries a small but documented risk of embryopathy (methimazole embryopathy syndrome). ### Beta-Blocker Role **Clinical Pearl:** Propranolol (not atenolol or other beta-blockers) is added because it: 1. Controls maternal tachycardia, palpitations, and anxiety 2. **Uniquely inhibits peripheral conversion of T4 to T3**, providing additional thyroid hormone suppression 3. Is safe in pregnancy (no teratogenicity at standard doses) Dose: Propranolol 40 mg twice or three times daily is standard. ### Dosing Strategy **Key Point:** PTU 100 mg three times daily (300 mg/day) is appropriate initial dosing for moderate-to-severe hyperthyroidism. Titrate based on free T4 levels; target is upper-normal range to avoid fetal hypothyroidism. ### Monitoring 1. Free T4 and TSH every 4–6 weeks 2. CBC at baseline and if symptoms of infection arise (agranulocytosis risk) 3. Liver function tests if symptoms of hepatotoxicity ### Why Not Methimazole? **Warning:** Although methimazole is preferred in non-pregnant patients (lower agranulocytosis risk, once-daily dosing), it is **contraindicated in the first trimester** due to the risk of methimazole embryopathy. This patient is 8 weeks pregnant (first trimester), so PTU is mandatory. ### Why Not Surgery? **Tip:** Thyroidectomy is reserved for: - Patients who cannot tolerate antithyroid drugs (agranulocytosis, hepatitis) - Failure of medical management - Very large goiters causing airway compression It is **not** first-line in pregnancy because it carries anesthetic and surgical risks. [cite:Harrison 21e Ch 397; Endocrine Society Clinical Practice Guidelines on Thyroid Disease in Pregnancy]