Carbimazole

• Graves' disease T2/T3
• Non-pregnant hyperthyroidism

Hydrolysed to methimazole → inhibits thyroid peroxidase → blocks iodination and coupling.

• Fetal aplasia cutis, choanal/oesophageal atresia in T1
• Switch PTU → carbimazole after 16 weeks
• Report sore throat immediately (agranulocytosis)

Start (moderate)

20–40 mg/day divided

Titration

Reduce to 5–15 mg OD by 4–8 weeks

Conversion

PTU 15 mg ≡ Carbimazole 1 mg

• AbsoluteFirst trimester (embryopathy)
• AbsolutePrevious carbimazole agranulocytosis or acute pancreatitis

• Agranulocytosis (~0.3%)
• Rash, urticaria
• Cholestatic hepatitis
• Acute pancreatitis (MHRA 2019)

• • T1 embryopathy risk (aplasia cutis, choanal atresia) — 2–4% MCM
• • Cross-titrate to PTU pre-conception or in T1, switch back at 16 weeks

• Warfarin

  Caution

  Anticoagulant effect changes with thyroid status

• Digoxin

  Caution

  Levels rise as thyroid normalises

• 💡 Baseline + regular FBC and LFTs
• 💡 Aim lowest dose to keep fT4 upper 1/3 of non-pregnant range