A 28-year-old woman from Mumbai presents with a 3-month history of palpitations, heat intolerance, and weight loss despite good appetite. On examination, she has a diffuse thyroid enlargement, tremor, and tachycardia (HR 110/min). TSH is <0.01 mIU/L, free T4 is 45 pmol/L (normal 10–20), and free T3 is 12 pmol/L (normal 3.5–8). She is started on propranolol and an antithyroid drug. Which antithyroid agent would you choose if she plans to conceive within 6 months?

Explanation

## Choice of Antithyroid Drug in Pregnancy Planning **Key Point:** Propylthiouracil (PTU) is the preferred antithyroid agent in the first trimester of pregnancy and in women planning conception within 6 months, due to a lower risk of embryopathy compared to methimazole. ### Mechanism of Teratogenicity Methimazole and carbimazole (which is metabolized to methimazole) are associated with **methimazole embryopathy**, characterized by: - Esophageal atresia - Choanal atresia - Cutis aplasia (scalp defects) - Developmental delay This risk is highest during the first trimester (organogenesis phase). ### Why PTU is Preferred 1. **Lower teratogenic risk** — PTU does not cross the placenta as readily and has not been associated with a recognizable embryopathy pattern. 2. **Timing consideration** — Since this patient plans conception within 6 months, she is in the preconception window when PTU should be initiated. 3. **Dual mechanism** — PTU not only inhibits thyroid peroxidase (TPO) but also inhibits peripheral conversion of T4 → T3, providing faster symptom relief. ### Comparative Antithyroid Drug Profile | Feature | PTU | Methimazole | Carbimazole | Iodine | |---------|-----|-------------|-------------|--------| | **Mechanism** | TPO inhibition + T4→T3 conversion block | TPO inhibition only | Prodrug of methimazole | Inhibits release + TPO | | **Teratogenicity** | Lower risk | Methimazole embryopathy | Same as methimazole | Not teratogenic | | **Onset** | 2–4 weeks | 2–4 weeks | 2–4 weeks | 3–7 days | | **Agranulocytosis risk** | 0.1–0.3% | 0.1–0.3% | 0.1–0.3% | None | | **Use in pregnancy** | 1st trimester preferred | Avoid 1st trimester | Avoid 1st trimester | Not used long-term | | **Duration** | Long-acting | Long-acting | Long-acting | Short-acting | **High-Yield:** PTU is the antithyroid of choice in preconception counseling and first-trimester pregnancy. Switch to methimazole after the first trimester if needed, as PTU carries a small risk of hepatotoxicity with prolonged use. ### Clinical Pearl Iodine solution (Lugol's iodine) works rapidly (3–7 days) and is safe in pregnancy, but it is reserved for acute thyroid storm or immediate preoperative preparation because prolonged use leads to escape phenomenon (tachyphylaxis) and iodine-induced hypothyroidism. ### Management Algorithm ```mermaid flowchart TD A[Woman with Graves' disease]:::outcome --> B{Planning pregnancy?}:::decision B -->|Yes, within 6 months| C[Start PTU]:::action B -->|No, or >1 year away| D[Start methimazole or PTU]:::action C --> E[Achieve euthyroid state]:::action E --> F[Conceive on PTU]:::action D --> G{Pregnant?}:::decision G -->|Yes, 1st trimester| H[Switch to PTU if on methimazole]:::action G -->|Yes, 2nd/3rd trimester| I[Can use methimazole]:::action ``` [cite:KD Tripathi 8e Ch 42]