Carbimazole

Carbimazole is an antithyroid drug that works by blocking thyroid peroxidase - the enzyme the thyroid uses to produce T4 and T3. By reducing hormone synthesis, it gradually brings thyroid levels back towards normal.

If you are outside the UK, you may know this drug by a different name: in the US and many other countries the equivalent antithyroid medicine is methimazole (also called thiamazole). Carbimazole is a prodrug that the body converts into methimazole, so the two are closely related but prescribed and dosed differently.

It is the first-choice antithyroid drug for most adults with hyperthyroidism in the UK, used for Graves' disease, toxic nodular goitre, and to prepare the thyroid before surgery or radioactive iodine. Treatment may be a course lasting 12 - 18 months (aiming for remission), or longer-term in people for whom other treatments are not appropriate.

Carbimazole is generally taken once or twice daily. Regular blood tests are needed in the early months to monitor thyroid levels and adjust the dose. The most serious - though uncommon - side effect is agranulocytosis: a dangerous drop in white blood cells. The safety advice your prescribing team gives is to stop carbimazole immediately and seek urgent help - attending A&E for an urgent blood count - if you develop a sore throat, fever, or mouth ulcers. This warning should be given to every person starting carbimazole.

Carbimazole is not recommended in the first trimester of pregnancy - propylthiouracil is preferred in early pregnancy.