About antithyroid drugs

Content overview

What are antithyroid drugs?

How are they used?

Will antithyroid drugs cure hyperthyroidism?

What are the side effects of taking antithyroid drugs?

Further reading

What are antithyroid drugs?

If you have been diagnosed with hyperthyroidism (overactive thyroid) your treatment will depend on several factors, such as the type of hyperthyroidism you have, and the severity of the thyroid disease.

Antithyroid drugs are often used as the first treatment and aim to reduce the production of thyroid hormones. They may be used short term to prepare for radioactive iodine treatment or surgery, or long term to provide a lasting cure for patients with Graves’ disease (a form of hyperthyroidism which may sometimes go into remission, either temporarily or  permanently, after a course of tablets). The first choice drug is Carbimazole (CMZ). If this causes you side effects, or if you are or plan to be pregnant, Propylthiouracil (PTU) is used.

All patients with a diagnosis of hyperthyroidism should be referred to a specialist doctor. This will usually be an endocrinologist who specialises in thyroid and other endocrine disorders. Antithyroid drugs are usually started on the recommendation of a specialist but sometimes your GP will prescribe them while you are waiting for an appointment.

How are they used?

You will initially be put on a high dose of either CMZ or, in specific circumstances, PTU. The overactive gland should be under control in about six to eight weeks.

Antithyroid drugs can also be used as part of a block and replace (BR) regimen: the antithyroid drug blocks the thyroid gland from producing any thyroxine. Levothyroxine is then given to replace your natural thyroxine.

Will antithyroid drugs cure hyperthyroidism?

A course of drug treatment lasting up to eighteen months gives you approximately a 30-50% chance of a cure, depending on for example the size of your goitre and how severe the overactivity is. It is important to continue to take your tablets every day as forgetting to take them will affect your blood test results and your health. You should not stop them unless advised by a doctor even if another illness develops. Smoking reduces the chance of a cure after a course of antithyroid drugs.

What are the side effects of taking antithyroid drugs?

There is a very rare and unpredictable side-effect of antithyroid drugs due to a lowering of the number of white blood cells, which is called agranulocytosis. Patients with low white blood cells may develop a sore throat, mouth ulcers, rash, or fever. Therefore, if you develop a sore throat, mouth ulcers, rash or an unexplained fever, stop taking the tablets immediately and have your white cell count checked urgently either through your GP or by going to your local Accident and Emergency department. In most cases, it turns out to be a false alarm and you can re-start your medication.

Very rarely, serious liver injury has been reported in patients, including children, taking PTU, especially during the first six months of taking the drug. Your doctor should monitor you for symptoms and discontinue the PTU if liver injury is suspected. If you notice any yellowing of the eyes or skin you should see your doctor immediately.

Further reading

Read 'Your Guide to Hyperthyroidism'

Read 'Your Guide to Antithyroid Drug Therapy to Treat Hyperthyroidism'

Read our antithyroid drugs alert card

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