What are antithyroid medicines?

Antithyroid medicines are used to treat hyperthyroidism (overactive thyroid) 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.

Are there different types of antithyroid medicines?

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

How do they work?

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 4 to 8 weeks of treatment for your thyroxine level to come down to normal.

Which is the best one?

Carbimazole is the most widely prescribed antithyroid drug 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.

How do I take antithyroid medicine?

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 4 to 8 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 drug 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 suffering an adverse effect from the higher dose of antithyroid medicine may be higher.

How quickly do antithyroid medicines work?

These medicines should have some effect on your symptoms around 10 to14 days after treatment starts. Thyroid hormone levels are usually stabilised within 4 to 8 weeks of taking the medication.

How long is treatment needed?

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 remit (get better) or relapse (get worse).

Medical evidence suggests that about half of people treated by the titration method will achieve remission (get better) 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.

Will hyperthyroidism return after treatment with antithyroid medicines?

As mentioned before, 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.

What would happen if I didn't take antithyroid medicines?

It is usually advisable to treat 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.

What are the possible side-effects?

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, which 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, if you develop a sore throat, mouth ulcers, unexplained bruising or bleeding, rash, fever or any other signs of infection then you must stop the medicine and report this to your doctor immediately.

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.

Can I buy them or do I need a prescription?

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.

Who can and cannot take antithyroid medicines?

Pregnant women or those planning a baby should seek the advice of their GP as these medicines are able to cross the placenta. 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.

Further help and information

A few self-help groups offer materials and local group support to people with hyperthyroidism; they include:

British Thyroid Foundation

2nd Floor, 3 Devonshire Place, Harrogate, HG1 4AA
Tel: 01423 709 707 Web: www.btf-thyroid.org

Thyroid Eye Disease Charitable Trust (TEDct)

PO BOX 2954, Calne, Wiltshire, SN11 8WR
Tel: 0844 800 8133 Web: www.tedct.co.uk

How to use the Yellow Card Scheme

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: http://yellowcard.mhra.gov.uk

The Yellow Card Scheme is used to make pharmacists, doctors and nurses aware of any new side-effects that your medicines 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.
• Information about 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.

References

• Hyperthyroidism, Prodigy (March 2008)
• Abraham P et al, Antithyroid drug regimen for treating Graves' hyperthyroidism (Cochrane Review), April 2009
• British National Formulary; 62nd Edition (Sep 2011) British Medical Association and Royal Pharmaceutical Society of Great Britain, London (link to current BNF)
• Agranulocytosis with anti-thyroid drugs. Current Problems in Pharmacovigilance, Medicines and Healthcare products Regulatory Agency (MHRA), Feb 1999