Methimazole (Carbimazole) — NZ Medication Guide
📋 Reviewed by a Registered Pharmacist NZ — This information is for general educational purposes only and does not replace professional medical advice. Always consult your doctor or pharmacist before starting, stopping, or changing any medication.
What is Methimazole (Carbimazole)?
Carbimazole (converted to methimazole) is an antithyroid medicine used to treat hyperthyroidism by reducing thyroid hormone production. This medication is funded by Pharmac for hyperthyroidism.
What is Methimazole (Carbimazole) Used For?
Carbimazole is used for hyperthyroidism (overactive thyroid), including Graves’ disease, toxic multinodular goitre, and toxic adenoma. It is the most commonly prescribed antithyroid drug in New Zealand.
How Does Methimazole (Carbimazole) Work?
Carbimazole is converted to methimazole in the body, which inhibits thyroid peroxidase — the enzyme responsible for iodinating thyroglobulin and producing thyroid hormones T3 and T4. By blocking this enzyme, carbimazole reduces circulating thyroid hormone levels.
How to Take Methimazole (Carbimazole)
Usually taken two to three times daily (or as a single daily dose with block-and-replace therapy). Starting doses range from 15–40 mg/day depending on severity. As thyroid levels normalise, the dose is reduced. Treatment typically continues for 12–18 months, with monitoring of thyroid function every 6–8 weeks initially.
Common Side Effects of Methimazole (Carbimazole)
- Rash and skin reactions (common — often mild)
- Nausea
- Joint pain
- Headache
- Hair loss
Serious Side Effects — Seek Medical Attention
Contact your doctor or call 111 immediately if you experience any of the following:
- Agranulocytosis (rare — 0.1–0.5% — potentially life-threatening; stop medicine immediately and seek emergency care if fever, sore throat, or mouth ulcers develop)
- Aplastic anaemia (rare)
- Liver toxicity (rare)
- Vasculitis (rare)
Drug Interactions
Methimazole (Carbimazole) may interact with other medicines. Always inform your doctor and pharmacist of all medications you are taking. Key interactions include:
- Warfarin (hyperthyroidism treatment may alter warfarin requirements — monitor INR closely)
- Digoxin (thyroid status affects digoxin requirements)
New Zealand Prescribing Information
Carbimazole (5 mg tablets) is funded by Pharmac. Patients must be counselled to seek immediate medical attention if they develop a fever, sore throat, or mouth ulcers — these may indicate agranulocytosis and require urgent FBC. A full blood count must be performed immediately and carbimazole stopped if agranulocytosis is confirmed. Propylthiouracil (PTU) is preferred in the first trimester of pregnancy.
Frequently Asked Questions
What should I do if I develop a fever or sore throat on carbimazole?
Stop taking carbimazole immediately and go to your nearest emergency department or urgent care clinic for a blood test (FBC). Fever and sore throat can be signs of agranulocytosis — a dangerous drop in white blood cells. Do not wait.
How long will I need to take carbimazole?
For Graves’ disease, treatment is typically continued for 12–18 months. Relapse is common (50–60%) after stopping. If you relapse, radioiodine therapy or thyroidectomy are definitive treatment options.
⚠️ Disclaimer: This page is for educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your doctor or a qualified health provider. In New Zealand, medication availability and funding may vary — check with Pharmac or your pharmacist for current information.
Reviewed by a Registered Pharmacist NZ
References & Further Information
The following New Zealand and international resources were used to inform this page: