Agranulocytosis in 1/1000 which can be fatal if undetected. Need to give written advice on any fever/sore throat to stop the drugs and get an urgent white cell count
- Is metabolised to methimazole the active drug
- Commonly used as first-line agent in the UK
- Inhibits iodination of tyrosine thus reducing T3 and T4
- May have some immunosuppressive activity
- Titration regimen:Dose Carbimazole 15-40 mg OD should cause euthyroidism within 2 weeks maximal at 2 months. Later reduce to 5-15 mg od po. Treat for 1-2 years.
- Block and replace : Carbimazole 40-60 mg daily and Thyroxine 100 mcg od for 18 months. Treatment failures should be considered for radioactive iodine
- Carbimazole 1 mg is considered equivalent to Propylthiouracil 10 mg
Dose range:You must check with BNF or drug datasheet
|Carbimazole (Block and replace. Give T4)||40-60 mg ||OD||PO. Lower dose in elderly|
|Carbimazole (Titrate)||15-40 mg ||OD||PO. Eventually reduce to 5-15 mg OD|
- Relative: Carbimazole in pregnancy crosses the placenta and can cause fetal hypothyroidism. With Breastfeeding, it may cause a neonatal goitre.
- Radioactive iodine is contraindicated in pregnancy so a risk/benefit assessment is needed and specialists may decide with patients to use carbimazole.
- Agranulocytosis in 1/300. Need to give written advice on any fever/sore throat to stop the drugs and get an urgent FBC
- Liver toxicity, Headache, rash, dyspepsia, Congenital malformations
Neutropenia and agranulocytosis
- Always be aware of the risk of bone marrow suppression induced by carbimazole
- If clinically suspected stop treatment promptly and send FBC.
- Patient must be told to report symptoms and signs suggestive of infection
- A key sign is a sore throat but also fever, cough, dyspnoea
- Check FBC and WCC if there is any clinical evidence of infection.
- Carbimazole should be stopped promptly if clinical or laboratory evidence of neutropenia.