An antithyroid medicine for an overactive thyroid
Methimazole (thiamazole)
An antithyroid medicine that reduces the production of thyroid hormone to control an overactive thyroid (hyperthyroidism).
What is Methimazole (thiamazole)?
Methimazole, also called thiamazole, is an antithyroid medicine that lowers the amount of thyroid hormone the gland makes, controlling an overactive thyroid (hyperthyroidism). In the UK the related prodrug carbimazole is usually prescribed and is converted to methimazole in the body. It is a prescription medicine that needs careful monitoring, because it can rarely affect the white blood cells that fight infection. Anyone taking it who develops a sore throat, fever or signs of infection should seek urgent medical advice and a blood test.
Education and reference only. This is a plain-language guide to Methimazole (thiamazole) — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Methimazole is a medicine used to treat hyperthyroidism, where the thyroid gland in the neck produces too much thyroid hormone and speeds up the body's processes. It reduces hormone production while other treatment decisions are made, such as whether the condition will settle, or whether radioactive iodine or surgery is needed. In the UK the same effect is usually achieved by prescribing carbimazole, which turns into methimazole inside the body. It is a prescription-only medicine and treatment is supervised by a doctor.
How it works
The thyroid gland uses iodine to build thyroid hormones. Methimazole blocks an enzyme involved in this process, so the gland makes less hormone. Over a few weeks, as the existing stores of hormone are used up, the symptoms of an overactive thyroid, such as a racing heart, weight loss, tremor and feeling hot and anxious, begin to settle. It does not destroy the gland; it simply slows hormone production while it is being taken, which is why treatment is continued and adjusted according to thyroid blood tests.
Company & origin
Originated / developed by: Various (carbimazole is the UK prodrug form).
An antithyroid drug used to calm an overactive thyroid gland. An important UK point: in Britain the related medicine carbimazole is normally prescribed, and the body converts carbimazole into methimazole (also called thiamazole) to do the work. Methimazole itself, under the international name thiamazole and the brand Tapazole, is used directly in the United States and parts of Europe. So a UK patient is most likely meeting this drug as carbimazole, while the active form acting in the body is methimazole.
What it treats
Conditions Methimazole (thiamazole) is used for
Practical use
How to take Methimazole (thiamazole)
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it exactly as prescribed and keep taking it even once you feel better, unless your doctor tells you to stop.
- Attend all blood-test appointments so your thyroid levels and blood counts can be checked.
- Seek urgent medical advice if you develop a sore throat, fever, mouth ulcers or other signs of infection.
- Tell your doctor straight away if you become pregnant or are planning a pregnancy, as treatment may need adjusting.
- Report any yellowing of the skin or eyes, dark urine, or persistent nausea, which can suggest liver effects.
Weighing it up
Advantages & disadvantages of Methimazole (thiamazole)
Advantages
- Effectively brings an overactive thyroid back under control without surgery.
- Can be adjusted and stopped, allowing some people's thyroid to settle on its own.
- Taken by mouth and managed in the community with blood-test monitoring.
Disadvantages
- Carries a rare but serious risk of suppressing infection-fighting white blood cells.
- Needs regular blood tests and can take several weeks to take full effect.
- Requires careful management in pregnancy because it can affect the developing baby.
Practical use
Good to know
The most important safety point is a rare but serious drop in infection-fighting white blood cells, called agranulocytosis. Because of this, anyone taking methimazole or carbimazole must seek urgent medical help and a blood count if they develop a sore throat, mouth ulcers, fever or other signs of infection. The medicine can also affect the liver. It needs careful handling in pregnancy, as it can affect the developing baby, so pregnancy and any plans for pregnancy should always be discussed with the specialist team.
Who should not take it / use with caution
- Anyone who has previously had a serious blood disorder caused by an antithyroid drug.
- People with severe existing liver problems, unless a specialist advises and monitors closely.
- Pregnant women or those planning pregnancy without specialist review, because of effects on the baby.
Monitoring
- Regular thyroid function blood tests to guide the dose.
- Urgent full blood count if any sign of infection develops.
- Liver checks and review of symptoms during treatment.
Side effects
- Itching, rash, joint aches or mild stomach upset, especially early on.
- Rarely, a serious fall in white blood cells (agranulocytosis), signalled by sore throat, fever or infection.
- Rarely, liver inflammation, signalled by jaundice, dark urine or persistent sickness.
Key interactions
- As thyroid levels normalise, the dose of other heart and blood-thinning medicines may need adjusting.
- Levels of medicines such as digoxin and certain beta-blockers can change as the thyroid settles.
- Tell every prescriber you take an antithyroid medicine so doses can be reviewed.
Available as: Tablets taken by mouth (in the UK most commonly prescribed as the related medicine carbimazole).
Answers
Methimazole (thiamazole): frequently asked questions
Why might I be given carbimazole instead of methimazole?
In the UK carbimazole is the standard antithyroid tablet. Once swallowed, the body converts carbimazole into methimazole, which is the form that actually reduces thyroid hormone production. They are closely related and do the same job.
What is the warning about sore throat and fever?
Very rarely these medicines can sharply lower the white blood cells that fight infection, a condition called agranulocytosis. A sore throat, fever or mouth ulcers can be the first sign, so you should seek urgent medical advice and a blood count rather than waiting.
How long until my thyroid symptoms improve?
Because the gland still holds stores of hormone, it usually takes a few weeks for symptoms such as a racing heart and tremor to settle. Your doctor may use other medicines to help with symptoms in the meantime.
Is it safe in pregnancy?
Antithyroid medicines can affect a developing baby, so pregnancy and any plans for pregnancy must be discussed with the specialist team. They will decide on the safest approach, which may mean changing the medicine or dose.
Will I need to take it forever?
Not always. Some people take it for a defined course to see whether their thyroid settles, while others need longer treatment or move on to radioactive iodine or surgery. The plan is decided with your specialist based on your blood tests and response.
Authoritative sources
- BNF
- NICE CKS
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