Thiamazole

Key facts

• Thiamazole, also called methimazole, is an antithyroid medicine that reduces the amount of thyroid hormone the thyroid gland produces. It is used to treat hyperthyroidism, most commonly caused by Graves' disease.
• It can rarely cause agranulocytosis, a sudden severe drop in infection-fighting white blood cells. If you develop a fever, sore throat, mouth ulcers or other signs of infection, stop the medicine and get an urgent blood count the same day.
• Thiamazole can harm a developing fetus, particularly in the first trimester, and is avoided during that time where possible; tell your prescriber immediately if you are or might be pregnant.
• Seek urgent care for fever with sore throat, yellowing of the skin or eyes, or dark urine.

What thiamazole treats

Thiamazole treats hyperthyroidism, an overactive thyroid gland producing too much thyroid hormone, most often due to Graves' disease or a toxic multinodular goitre. It is also used before thyroid surgery or radioactive iodine treatment to bring thyroid hormone levels down to a safer range beforehand.

How thiamazole works

The thyroid gland uses an enzyme called thyroid peroxidase to attach iodine to a protein and build thyroid hormone. Thiamazole blocks thyroid peroxidase, so the gland cannot make new thyroid hormone. It does not remove hormone already stored in the gland, so it typically takes several weeks of treatment before hormone levels and symptoms improve.

Before you take it

• Do not take thiamazole if you have had a serious blood reaction (agranulocytosis) to it or a related antithyroid drug before.
• Tell your prescriber if you are pregnant or planning pregnancy; thiamazole is generally avoided in the first trimester because of a risk of birth defects, and treatment is reviewed carefully if you are breastfeeding.
• Tell your prescriber about other medicines, since thiamazole can affect the dose needed of blood thinners such as warfarin.
• A baseline full blood count is usually checked before starting, so any later drop can be identified against it.

Side effects

Common effects include skin rash, itching, joint pain, nausea and a mild, temporary reduction in white blood cells.

Stop and seek urgent medical care for any of these:

• Fever, sore throat, mouth ulcers or other signs of infection, which need an urgent blood count.
• Yellowing of the skin or eyes, dark urine or pale stools.
• Severe joint pain or swelling.
• Unusual bruising or bleeding.

Safety essentials

• Agranulocytosis is the defining risk of thiamazole: report fever or sore throat immediately, stop the medicine and get an urgent blood count, since this reaction can develop suddenly even after months of uneventful treatment.
• Thiamazole is a known teratogen and is avoided in the first trimester of pregnancy where possible; tell your prescriber straight away if you become pregnant while taking it.
• Do not stop thiamazole abruptly without medical advice, since untreated hyperthyroidism can also cause serious harm; any change in dose should be supervised.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.