Revised 2018

Hyperthyroidism is a condition where the thyroid produces more thyroid hormone than is needed by the body. It is also known as an over-active thyroid or thyrotoxicosis.

The Drugs

  • Carbimazole (CMZ) is normally prescribed in the first instance
  • Propylthiouracil (PTU) is an alternative and is often used pre-pregnancy, in early pregnancy or if CMZ cannot be tolerated
  • Both drugs reduce the amount of thyroid hormone released into the blood
  • PTU is not recommended for children unless they are allergic to CMZ

Initial Treatment

  • Initially a high dose of an antithyroid drug
  • Your doctor will review the dose after about 3 - 8 weeks
  • A blood test will be done on review to check your thyroid levels
  • The drug dose may be adjusted if there is improvement

Are there any side effects?

  • Sometimes minor ones, such as nausea, altered taste sensation or a rash
  • Very rarely, a serious side effect called agranulocytosis where the white blood cell count may be lowered. Watch out for signs of sore throat, unexplained fever or mouth ulcers and if you notice any of these go to your GP or nearest Accident and Emergency department to request a full blood count and do not take another tablet until told it is safe to do so
  • Very rarely, PTU can cause serious liver injury. If you notice yellowing of the eyes or skin, go to see a doctor immediately and ask for a liver enzyme test

How long will I need treatment?

  • This will normally depend on the cause of your hyperthyroidism and other factors such as age.
  • You may be able to stop treatment within 12-18 months if your thyroid levels are within the normal range
  • You may be prescribed antithyroid drugs for just a few weeks to bring your thyroid levels within the normal range and then treated with radioactive iodine or surgery
  • A course of drug treatment lasting up to eighteen months gives you approximately a 30-50% chance of a cure, depending on the size of the goitre (an enlarged thyroid gland) and the severity of the over-activity. Some elderly patients may be prescribed anti-thyroid drugs for life
  • While you are taking anti-thyroid drugs the dose may need adjusting
  • Blood tests and check-ups will be carried out regularly over the next six to 12 months to check your thyroid levels
  • You will need further blood tests to check your thyroid levels if your symptoms return
  • Smokers are up to three times more likely to suffer a recurrence than non-smokers

What if the hyperthyroidism returns?

Treatment options are:

  • Re-start the anti-thyroid drugs
  • Surgery to remove all or part of the thyroid gland
  • Radioactive iodine which usually destroys most of the thyroid tissue

Antithyroid drugs and pregnancy

If you are pregnant, or planning to have a baby, you should tell your doctor and you may need to adjust your medication and to have more frequent blood tests.

Thyroid problems often run in families and if family members are unwell they should be encouraged to discuss with their own GP whether thyroid testing is warranted.

This Quick Guide is one in a series about thyroid disorders. All Quick Guides are available on the British Thyroid Foundation website.

A leaflet containing more detailed information about Antithyroid Drug Therapy is also available.

The British Thyroid Foundation
tel: 01423 810093
The British Thyroid Foundation is a registered charity: England and Wales No 1006391, Scotland SC046037

Membership Information
Quick Guides

Endorsed by:

The British Thyroid Association - medical professionals encouraging the highest standards in patient care and research

The British Association of Endocrine and Thyroid Surgeons - the representative body of British surgeons who have a specialist interest in surgery of the endocrine glands (thyroid, parathyroid and adrenal)

First issued: 2008
Revised: 2010, 2011, 2015, 2018
Our literature is reviewed every two years and revised if necessary.

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