Almondsbury Surgery

Almondsbury Surgery Sundays Hill Almondsbury BS32 4DS

Health Information

Antithyroid Medicines

Antithyroid Medicines

Antithyroid medicines are used to treat overactive thyroid (hyperthyroidism). Carbimazole is the most commonly used. You may require careful monitoring to get the right levels of these medicines for you. Most of the side-effects experienced are minor. However, when taking an antithyroid medicine, if you develop any of the side-effects (listed below) or any other signs of infection, you must stop the medicine and report this to your doctor immediately.

Antithyroid medicines are used to treat overactive thyroid (hyperthyroidism), also known as thyrotoxicosis. There are different causes of hyperthyroidism. The causes of hyperthyroidism where antithyroid medicines are used include:

  • Graves' disease - the most common cause of hyperthyroidism.
  • Severe hyperthyroidism - known as thyrotoxic crisis or thyroid storm.
  • In some people with thyroid nodules - lumps on the thyroid gland which may release thyroid hormones.
  • The treatment of some forms of cancer.

The most commonly used antithyroid medicine in the UK is carbimazole, followed by propylthiouracil. Carbimazole and propylthiouracil belong to a group (class) of medicines called thionamides. Thionamides have similar actions on the thyroid gland.

Thyroxine (also known as T4) is a body chemical (hormone) made by the thyroid gland. It is carried round the body in the bloodstream. It helps to keep the body's functions (the metabolism) working at the correct pace. Many cells and tissues in the body need thyroxine to keep them working correctly.

'Hyperthyroidism' means an overactive thyroid gland. When the thyroid gland is overactive it makes too much thyroxine. The extra thyroxine causes many of the body's functions to speed up. (In contrast, if you have hypothyroidism, you make too little thyroxine. This causes many of the body's functions to slow down.)

Thionamides such as carbimazole reduce the amount of hormone released by the thyroid gland. Carbimazole does not affect the thyroxine which is already made and stored, but reduces further production. Therefore, it may take four to eight weeks of treatment for your thyroxine level to come down to normal.

Carbimazole is the most widely prescribed antithyroid medicine in the UK. Propylthiouracil can be used instead if you develop a side-effect to carbimazole, or it may be used in a thyrotoxic crisis. Your doctor will advise you on which is the most suitable for you.

Getting the right balance of thyroid hormone in your blood can take time.

Doctors have two main methods of trying to get the balance right. The first involves taking an initial high dose of carbimazole to reduce the amount of thyroxine in your blood. This dose is taken until the hormone levels in your blood have stabilised, usually about four to eight weeks later. Because your body needs a certain amount of thyroxine to function properly, the high dose is then slowly decreased. Usually, your hormone levels will be checked by a blood test every month or so. And the dose of medicine you are taking will be changed depending upon the results of your thyroid hormone levels. The aim of this treatment is to keep you on the lowest level of antithyroid medicine necessary. This treatment method is called 'titration'.

It can be difficult for a doctor to judge just the right dose of carbimazole to give in each case. Too much treatment may make the thyroxine level go too low. Not enough treatment means the level remains higher than normal. This is the reason for the regular blood tests and careful monitoring.

The second option is to deliberately take a high dose of carbimazole each day. This stops the thyroid gland making any thyroxine. Your doctor can then prescribe a daily dose of thyroxine to keep the blood level of thyroxine normal. This 'over-treatment' coupled with taking replacement thyroxine is called 'block and replace' and is a popular option.

It is generally thought that the 'block and replace' method results in better control of thyroid hormone levels. However, the risk of experiencing an adverse effect from the higher dose of antithyroid medicine may be higher.

These medicines should have some effect on your symptoms around 10 to 14 days after treatment starts. Thyroid hormone levels are usually stabilised within four to eight weeks of taking the medication.

This may vary depending on the way in which you take your medicines. Hyperthyroidism is what is known as a relapsing-remitting illness. This means that the symptoms of the condition may get better (remit) or get worse (relapse).

Medical evidence suggests that about half of people treated by the titration method will get better (achieve remission) after 18 to 24 months of treatment. However, about half of those treated by the 'block and replace' method will achieve remission within six months of treatment. Your doctor will advise you on which treatment option may be suitable for you. 'Block and replace' therapy is not suitable in pregnancy.

As mentioned before, overactive thyroid (hyperthyroidism) is generally a relapsing-remitting illness, which means symptoms may return after treatment. If you feel unwell following treatment you should return to your doctor. Your GP should be able to advise you on the type of symptoms to look out for.

It is usually advisable to treat overactive thyroid (hyperthyroidism). Untreated hyperthyroidism can cause significant problems with your heart and other organs. It may also increase your risk of complications should you become pregnant. However, in many cases there are other treatment options. That is, radioactive iodine or surgery may be suitable options.

See separate leaflet called Hyperthyroidism (Overactive Thyroid) for details of these other treatment options.

Most people who take antithyroid medicines do not experience any side-effects. The side-effects that most commonly occur are:

  • Rash
  • Pruritus (itching)
  • Mild stomach upset
  • Headache
  • Painful joints

The above side-effects are usually not serious and often go, even if you continue with the medication.

A rare but serious side-effect is an effect on the blood-making cells. This can drastically reduce the number of blood cells in your body, including the cells that fight off infection and those that help to stop bleeding. Therefore, you must stop the medicine and report this to your doctor immediately, if you develop:

  • A sore throat.
  • Mouth ulcers.
  • Unexplained bruising or bleeding.
  • Rash.
  • High temperature (fever).
  • Any other signs of infection.

As noted above, a mild rash is a common side-effect. The rash associated with this rare but serious effect on blood-making cells is different. Therefore, when taking an antithyroid medicine, always report a rash to a doctor who can then decide if it is a common and minor problem or the more serious rash.

You cannot buy these medicines. They are only available from your chemist, with a doctor's prescription, and are usually started by a specialist doctor.

Pregnant women or those planning a baby should seek the advice of their GP, as these medicines are able to cross the placenta. The placenta is the organ that provides nourishment and oxygen to a baby in the womb (uterus). Antithyroid medicines may not be suitable for people with some forms of liver or kidney disease.

A full list of people who should not take antithyroid medicines is included with the information leaflet that comes with your medicine. Read this to be sure you are safe to take it.

These medicines sometimes react with other medicines that you may take. So, make sure your doctor knows of any other medicines that you are taking, including ones that you have bought rather than been prescribed.

If you think you have had a side-effect to one of your medicines, you can report this on the Yellow Card Scheme. You can do this online at the following web address:

The Yellow Card Scheme is used to make pharmacists, doctors and nurses aware of any new side-effects that your medicines or any other healthcare products may have caused. If you wish to report a side-effect, you will need to provide basic information about:

  • The side-effect.
  • The name of the medicine which you think caused it.
  • The person who had the side-effect.
  • Your contact details as the reporter of the side-effect.

It is helpful if you have your medication and/or the leaflet that came with it with you while you fill out the report.

Further help & information

British Thyroid Foundation

Suite 12, One Sceptre House, North Hornbeam Park, Hornbeam Square, Harrogate, HG2 8PB

Tel: 01423 810093

TEDct - Thyroid Eye Disease Charitable Trust

PO Box 1928, Bristol, BS37 0AX

Tel: 0844 800 8133

Thyroid UK

32 Darcy Road, St Osyth, Clacton On Sea, Essex, CO16 8QF

Tel: 01255 820407

Further reading & references

  • Hyperthyroidism; NICE CKS, June 2013 (UK access only)
  • Abraham P et al, Antithyroid drug regimen for treating Graves' hyperthyroidism (Cochrane Review), April 2009
  • Taylor PN, Vaidya B; Side effects of anti-thyroid drugs and their impact on the choice of treatment for thyrotoxicosis in pregnancy. Eur Thyroid J. 2012 Oct;1(3):176-85. doi: 10.1159/000342920. Epub 2012 Sep 24.
  • British National Formulary

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but makes no warranty as to its accuracy. Consult a doctor or other healthcare professional for diagnosis and treatment of medical conditions. For details see our conditions.

Original Author:
Dr Tim Kenny
Current Version:
Dr Colin Tidy
Peer Reviewer:
Dr John Cox
Document ID:
9302 (v3)
Last Checked:
Next Review: