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Pregnant lady in 2nd trimester has hyperthyroidism which drug we have to give

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hyperthyroidism pregnancy antithyroid drug management

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Multiple authoritative textbooks are fully consistent. Here is the complete answer:

Hyperthyroidism in Pregnancy - 2nd Trimester

Drug of Choice: Methimazole (MMI) / Carbimazole

The antithyroid drug used depends on the trimester:
TrimesterDrug of ChoiceReason
1st trimesterPTU (Propylthiouracil)Methimazole is teratogenic during organogenesis (causes methimazole embryopathy - choanal atresia, esophageal atresia, aplasia cutis, umbilical defects)
2nd & 3rd trimesterMethimazole (MMI) / CarbimazolePTU carries risk of hepatotoxicity; MMI is preferred post-organogenesis

Why the Switch at 2nd Trimester?

  • PTU is avoided in the 2nd and 3rd trimesters because of its risk of serious hepatotoxicity (maternal liver failure)
  • Methimazole is safe to use after organogenesis is complete, and has a longer duration of action, making it more convenient (once daily dosing)
  • Typical doses: Methimazole 10-20 mg/day (lowest effective dose)

Key Management Principles

  1. Goal of therapy: Keep free T4 (fT4) within or slightly above the trimester-specific reference interval - do NOT over-treat
  2. Minimum effective dose: Antithyroid drugs cross the placenta and can cause fetal hypothyroidism and goiter - use the lowest possible dose
  3. Block-and-replace regimen is CONTRAINDICATED in pregnancy - it requires higher antithyroid doses and does not prevent neonatal hypothyroidism
  4. Radioactive iodine (131I) is absolutely contraindicated throughout all trimesters - it destroys the fetal thyroid gland
  5. Beta-blockers (e.g., propranolol) may be added short-term for symptomatic control (tremor, palpitations) pending disease control
  6. Iodide is Class D in pregnancy - use only in severe/emergent cases for limited duration (days only)
  7. Thyroidectomy is rarely indicated; if needed, it is safest in the 2nd trimester

Monitoring

  • Monitor fetal thyroid gland size by ultrasonography (antithyroid drugs cross the placenta)
  • Monitor maternal TFTs (TSH, fT4) regularly
  • For Graves' disease: monitor TRAb (TSH receptor antibodies) - up to 20% of neonates of TRAb-positive mothers develop transient neonatal hyperthyroidism lasting 3-12 weeks
Sources: Rosen's Emergency Medicine (9th ed.); Tietz Textbook of Laboratory Medicine (7th ed.); Tintinalli's Emergency Medicine; Goldman-Cecil Medicine
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