Carbimazole 💊
WRITTEN WARNING MUST BE GIVEN to patients prescribed Carbimazole so they are aware to seek immediate medical help if they experience any signs or symptoms of agranulocytosis, such as a sore throat or mouth ulcers. Agranulocytosis is a serious condition affecting about 0.3 in 1000 patients. It should be suspected in patients on high doses of Carbimazole who present with symptoms of infection, such as a sore throat or mouth ulcers. The patient should stop taking the drug immediately and seek medical help. The risk of agranulocytosis is lower with Carbimazole than with Propylthiouracil. Once agranulocytosis is confirmed, it is an absolute contraindication to further antithyroid drug treatment, and radio-iodine therapy should be considered.
- 🔹 Prodrug, metabolised to methimazole (active form).
- 📌 Commonly used as the first-line antithyroid agent in the UK.
⚙️ Action
- 🚫 Inhibits iodination of tyrosine residues, reducing T3 and T4 synthesis.
- 🛡️ May also have mild immunosuppressive activity.
🎯 Indications / Dose
- 📉 Titration regimen:
Carbimazole 15–40 mg PO OD → should achieve euthyroidism within ~2 weeks (max at 2 months).
Then reduce to 5–15 mg PO OD. Continue for 12–24 months.
- 🔄 Block and replace:
Carbimazole 40–60 mg PO daily + Thyroxine 100 mcg OD for 12–18 months.
If relapse → consider radioactive iodine therapy.
- 📌 Drug equivalence: Carbimazole 1 mg ≈ Propylthiouracil 10 mg.
⛔ Contraindications
- 🚫 Pregnancy: Carbimazole crosses placenta → risk of fetal hypothyroidism and congenital malformations.
➡️ Radioactive iodine is contraindicated in pregnancy, so specialist-led risk/benefit decisions are needed.
- ⚠️ Breastfeeding: may cause neonatal goitre; use lowest effective dose if unavoidable.
🔄 Interactions
- ⚠️ Concomitant drugs affecting bone marrow (e.g. clozapine) ↑ risk of agranulocytosis.
- 📉 May alter response to warfarin (monitor INR closely).
⚠️ Side Effects
- 🧪 Agranulocytosis / neutropenia (rare but serious – see below).
- 🩸 Liver toxicity.
- 🤕 Headache, rash, dyspepsia.
- 👶 Congenital malformations if used in pregnancy (dose-dependent risk).
🧾 Neutropenia & Agranulocytosis
- ⚠️ Always consider bone marrow suppression in a patient on carbimazole.
- ❗ If clinically suspected → stop treatment immediately and send urgent FBC/WCC.
- 📝 Patients must be counselled to report:
– Sore throat
– Fever
– Cough
– Dyspnoea
- 🚫 Discontinue carbimazole if neutropenia is confirmed clinically or on labs.
📚 References
