Carbimazole for an overactive thyroid

Carbimazole is the tablet most commonly used to treat an overactive thyroid. Usually it is started at quite a high dose, until the thyroid blood tests settle down. You should begin to feel the benefit from carbimazole treatment after 2-3 weeks, but it can take 4-6 weeks for the blood tests to come back to normal.

Carbimazole is often used to begin with, to control the overproduction of thyroid hormone, while you have further tests to work out what caused your thyroid to become overactive in the first place. Beta blocker tablets, such as propranolol, may also be used if you have severe symptoms, while the thyroid overactivity is being brought under control. Beta blockers are not recommended if you have asthma.

If your doctor confirms that the cause of your overactive thyroid is Graves disease, a longer-term treatment option is to continue the carbimazole tablets for 12 to 18 months. As your thyroid blood tests settle, your doctor may either reduce the dose of carbimazole, or add in thyroxine (which is a thyroid hormone tablet) so that your thyroid blood tests do not go underactive. After a full course of 12-18 months of tablet treatment, the thyroid function will remain normal in about 50% of people. In the other 50%, their thyroid will become overactive again at some stage.

Unfortunately it is not possible to predict whether your Graves disease will be cured by tablet treatment or not. If your overactive thyroid is caused by thyroid nodules, then carbimazole treatment will control the over-production of thyroid hormones, but will not cure the nodules.

Carbimazole is a very safe medicine which has been used for many years. The commonest side effect is a rash, which may settle with antihistamine treatment. Other side effects are rare. A very rare but serious side effect of carbimazole treatment is reduced production of white blood cells by the bone marrow. This makes you more vulnerable to infections. The first sign that this might be happening may be a very sore mouth or throat, with severe ulcers, for no other obvious reason. If this happens, you must stop taking the tablets immediately and see a doctor the same day for a blood test to measure your white blood cell count.

There are other ways of treating an over active thyroid, including radioiodine treatment or surgery. We will discuss these separately. Some people opt for a course of medical treatment first, then radioiodine or surgery if their thyroid becomes overactive again afterwards. Other people do not want to take tablets for 12 to 18 months if there is only a 50% chance of being cured, and would rather have radioiodine or surgery.

If you are pregnant, trying for a baby, or breast feeding, you should tell your doctor, as this information is important in relation to your treatment plan.

This information is intended for patients receiving care in Brighton & Hove or Haywards Heath.

The information in this leaflet is for guidance purposes only and is in no way intended to replace professional clinical advice by a qualified practitioner.