## Antithyroid Drug Selection in Pregnancy ### Trimester-Based Approach ```mermaid flowchart TD A[Pregnant woman with Graves' disease]:::outcome --> B{Trimester?}:::decision B -->|1st trimester| C[PTU preferred]:::action C --> D[Avoid methimazole: teratogenic]:::urgent B -->|2nd & 3rd trimester| E[Switch to methimazole]:::action E --> F[Lower transplacental passage]:::outcome B -->|Postpartum/Lactation| G[Methimazole preferred]:::action G --> H[Better compliance, longer half-life]:::outcome ``` **Key Point:** PTU is preferred in the **first trimester** because methimazole is associated with **methimazole embryopathy** (rare but serious: methimazole-induced cutis aplasia, choanal/esophageal atresia, and developmental delay). **High-Yield:** After the first trimester, **switch to methimazole** because: - PTU carries a risk of **hepatotoxicity** (including fulminant hepatic failure) with prolonged use - Methimazole has lower transplacental passage in later pregnancy - Methimazole has a longer half-life (5–6 hours vs PTU 1–2 hours), improving compliance ### Adverse Effects: Both Drugs | Adverse Effect | PTU | Methimazole | Timing | |---|---|---|---| | Agranulocytosis | ~0.2–0.3% | ~0.3% | Usually first 3 months | | Hepatotoxicity | Fulminant (rare) | Mild LFT elevation | Variable | | Rash/Pruritus | Common | Common | Early | | Arthralgia | Yes | Yes | Variable | | Teratogenicity | No | Yes (embryopathy) | 1st trimester | **Key Point:** Agranulocytosis is an **idiosyncratic** (not dose-dependent) adverse effect occurring in ~0.2–0.3% of patients, typically within the **first 3 months** of therapy. Patients must be counseled to report fever, sore throat, or mouth ulcers immediately. ### Why Option 3 is Incorrect **PTU is NOT completely safe in pregnancy.** While it is preferred in the first trimester due to lower teratogenic risk, PTU carries significant risks: 1. **Hepatotoxicity**: Prolonged PTU use is associated with liver injury, including fulminant hepatic failure and hepatic necrosis. This risk increases with duration of therapy. 2. **Neonatal complications**: Even PTU crosses the placenta and can cause neonatal hypothyroidism and goiter if maternal dose is excessive. 3. **Lactation**: PTU is excreted in breast milk (though in small amounts). While generally considered safer than methimazole in lactation, it is not "completely safe." **Clinical Pearl:** The modern approach is to use PTU only in the **first trimester**, then switch to methimazole for the remainder of pregnancy and postpartum to minimize PTU hepatotoxicity risk while avoiding methimazole teratogenicity. [cite:Harrison 21e Ch 405]
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