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Antithyroid Drug Pregnancy: D in first trimester — switch to propylthiouracil

Carbimazole

Brand names: Neo-Mercazole

Adult dose

Dose: 15–40mg once daily (titration regimen) or block-replace regimen: 40mg OD + levothyroxine
Route: Oral
Frequency: Once or divided doses
Titration: start 20–40mg OD, reduce when euthyroid (usually 4–8 weeks) to maintenance 5–15mg OD. Block-replace: 40mg + LT4 for 6–12 months.

Clinical pearls

  • URGENT: advise patient to STOP and seek IMMEDIATE medical attention if fever, sore throat, or mouth ulcers — agranulocytosis
  • Active metabolite methimazole — more potent than propylthiouracil
  • Propylthiouracil preferred in first trimester pregnancy (carbimazole → aplasia cutis risk)
  • Titration vs block-replace: evidence suggests similar outcomes, block-replace requires less frequent monitoring
  • Radioiodine or surgery after 12–18 months of antithyroid therapy is definitive treatment

Contraindications

  • Severe agranulocytosis
  • Pregnancy in first trimester (use propylthiouracil instead)

Side effects

  • Agranulocytosis (0.3% — STOP immediately if fever/sore throat)
  • Rash
  • Arthralgia
  • Hepatitis
  • Aplastic anaemia

Interactions

  • Warfarin — hyperthyroidism correction increases warfarin requirement (monitor INR closely)

Monitoring

  • TFTs (every 4–8 weeks until stable, then 3-monthly)
  • FBC if fever/sore throat (urgent neutrophil count)
  • LFTs (baseline and if symptomatic)

Reference: BTA Hyperthyroidism Guidelines 2019. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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