Anti-thyroid Drugs

What are anti-thyroid drugs?

Anti-thyroid drugs are used to treat hyperthyroidism (overactive thyroid) also known as thyrotoxicosis. There are different causes of hyperthyroidism. The causes of hyperthyroidism where anti-thyroid drugs are used include:

• Graves’ disease – the commonest cause of hyperthyroidism.
• In 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.
• In the treatment of some forms of cancer.

Are there different types of anti-thyroid drugs?

The most commonly used anti-thyroid drug in the UK is carbimazole, followed by propylthiouracil. Carbimazole and propylthiouracil belong to a class (group) of drugs 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 anti-thyroid 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 anti-thyroid drugs?

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 – 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 altered in relation to the results of your thyroid hormone levels. The aim of this treatment is to keep you on the lowest level of anti-thyroid drug 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 anti-thyroid drug may be higher.

How quickly do anti-thyroid drugs work?

These drugs should have some effect on your symptoms around 10-14 days after treatment starts. Thyroid hormone levels are usually stabilised within 4-8 weeks of taking the medication.

How long is treatment needed?

This may vary depending on the way in which you take your drugs. 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 18-24 months 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 anti-thyroid drugs?

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 anti-thyroid drugs?

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.

Who can and cannot take anti-thyroid drugs?

Pregnant women or those planning a baby should seek the advice of their GP as these drugs are able to cross the placenta. Anti-thyroid drugs may not be suitable for people with some forms of liver or kidney disease.

A full list of people who should not take anti-thyroid drugs is included with the information leaflet that comes in the drug packet. Read this to be sure you are safe to take it.

What about side-effects?

Most people who take anti-thyroid drugs 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.

Serious note: 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 drug 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 affect on blood-making cells is different. Therefore, when taking an anti-thyroid drug 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?

These drugs are prescription only and are usually started by a specialist doctor.