Antithyroid Drug Pregnancy: Crosses the placenta and is suspected to cause congenital malformations, particularly in the first trimester and at high doses (reported malformations include aplasia cutis congenita, choanal atresia, craniofacial malformations, exomphalos, oesophageal atresia and ventricular septal defect). Should be used in pregnancy only when propylthiouracil is not suitable, and only at the lowest effective dose without additional thyroid hormones; the blocking-replacement regimen should not be used in pregnancy. Women of childbearing potential must use effective contraception during treatment. Secreted in breast milk - the patient should not breast-feed if treatment is continued during lactation.

Carbimazole

Brand names: Neo-Mercazole

Used in: Thyroid Disorders

Carbimazole is an antithyroid drug used to treat hyperthyroidism, particularly Graves' disease, to restore euthyroidism or to prepare for definitive treatment.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Initial dose 20 mg to 60 mg daily

Route: oral

Frequency: taken as two to three divided doses

Should only be administered if hyperthyroidism has been confirmed by laboratory tests. The dose is titrated against thyroid function until the patient is euthyroid to reduce the risk of over-treatment and resultant hypothyroidism. Subsequent therapy by one of two regimens: (1) Maintenance regimen - final dosage usually 5 mg to 15 mg per day, which may be taken as a single daily dose; (2) Blocking-replacement regimen - dosage maintained at initial level (20-60 mg/day) with supplemental L-thyroxine 50-150 micrograms/day given concomitantly to prevent hypothyroidism. Therapy should be continued for at least six months and up to 18 months with serial thyroid function monitoring. Elderly: no special dosage regimen required, but risk of fatal neutrophil dyscrasia may be greater in those aged 65 or over.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to the active substance or to any of the excipients
Serious, pre-existing haematological conditions
Severe hepatic insufficiency
History of acute pancreatitis after administration of carbimazole or its active metabolite thiamazole

Side effects

Nausea, mild gastrointestinal disturbance
Headache, arthralgia
Skin rashes, pruritus, urticaria
Bone marrow depression including neutropenia, eosinophilia, leucopenia and agranulocytosis (fatalities with carbimazole-induced agranulocytosis have been reported)
Hepatic disorders including abnormal liver function tests, hepatitis, cholestatic jaundice

Interactions

There is a risk of cross-allergy between carbimazole, the active metabolite thiamazole (methimazole) and propylthiouracil
Carbimazole should be stopped temporarily at the time of administration of radio-iodine (to avoid thyroid crisis)
Supplemental L-thyroxine may be co-administered in the blocking-replacement regimen

Clinical monograph

How it works

It is converted to methimazole, which inhibits thyroid peroxidase and so reduces thyroid hormone synthesis.

Prescribing in practice

Agranulocytosis is the key risk — advise stopping and seeking an urgent full blood count for sore throat, fever, mouth ulcers or other infection.
It is teratogenic (especially the first trimester) — use effective contraception and seek specialist advice in pregnancy (propylthiouracil may be preferred early).
Its effect on thyroid levels is delayed; a beta-blocker is often added initially for symptom control.

Monitoring

Monitor thyroid function to titrate the dose; check FBC urgently with infection symptoms, and review liver function if hepatotoxicity is suspected.

Counselling the patient

Stop it and get an urgent blood test if you develop a sore throat, fever or mouth ulcers.
It takes some weeks to control symptoms.
Use reliable contraception and tell your clinician if you might be pregnant.

Evidence & guidelines

First-line antithyroid drug for hyperthyroidism, with mandatory counselling about agranulocytosis.

Reference: BTA Hyperthyroidism Guidelines 2019; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways