What is hyperthyroidism?

Hyperthyroidism is a condition where the thyroid produces more thyroid hormones than is needed by the body. It is also referred to as thyrotoxicosis, or an over-active thyroid.

What drugs are used to treat hyperthyroidism?

There are two antithyroid drugs commonly used in the UK: carbimazole or Neo Mercazole (CMZ) and propylthiouracil (PTU). They both work by reducing the amount of thyroid hormone released into the circulation. They may be used short term to prepare for radioactive iodine treatment or surgery, or long term with the aim of a lasting cure for patients with Graves’ disease (a form of hyperthyroidism which may sometimes be cured after a course of tablets). They are the treatment of choice for women who are pregnant and the usual starting treatment for children.

It is recommended that all patients with a diagnosis of hyperthyroidism are referred to a specialist (usually 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.

What is the course of treatment?

Initially you are started on a high dose of either CMZ or (in some specific circumstances) PTU. The over-active gland should be under control in about six to eight weeks.

You should visit your doctor a few weeks after you start the therapy to see how your body is adjusting and to repeat your thyroid function blood test. If things are improving the medication may be adjusted in one of two ways:

  • Titration: The dose of the drug is reduced, the aim being to keep you on the lowest dose of the drug needed for your thyroid function to be normal (euthyroid).
  • Block and replace: You continue taking CMZ, usually 20-40mg daily, or PTU, usually 200-400mg daily, to stop your thyroid gland producing thyroid hormone; and start taking levothyroxine (usually 100-150mcg daily) to replace the thyroid hormone your body would normally produce. Block and replace must not be used in pregnancy as the high doses of antithyroid drugs cross the placenta and can cause the baby to develop an under-active thyroid.

You will usually continue on antithyroid drugs alone for up to 18 months, and on block and replace therapy for six to 12 months. If you have Graves’ disease there is about a one in three chance that you will have no further problems with your thyroid after a single course of antithyroid drugs. You will have regular blood tests and check-ups over the next six to 12 months in case your thyroid gland becomes over-active again (this is known as a relapse). After 12 months the risk of a relapse is low but relapses can occur many months or years after the first episode.

Provided you are free of symptoms and your thyroid blood test remains normal one year after treatment you will need no further check-ups other than occasional thyroid blood tests. It is, however, important to see your GP and to ask for a blood test if you notice any symptoms of hyperthyroidism in the future.

Who is more likely to relapse?

It is not possible to predict this with any certainty, but if there have been problems controlling your hyperthyroidism with drugs or if you have a very large thyroid gland then there is more chance of continuing problems. Women and those over the age of 40 seem less likely to relapse after a course of treatment.

Does it matter which drug I am put on?

Most doctors prescribe CMZ in the first instance.

CMZ controls the over-active gland more rapidly than PTU and is more convenient to take as it can be taken once daily, at least once the over-active thyroid gland is under control. Usually a smaller number of tablets is needed than with PTU. CMZ is available in 5mg and 20mg tablets, so a dose of (for example) 40mg daily requires only two tablets once a day.

PTU comes only in 50mg tablets and is usually taken two or three times a day, so an equivalent PTU dose of 400mg daily would require eight tablets divided over two or three doses. PTU is usually used if you are intolerant to CMZ. It is the treatment of choice during the first three months of pregnancy, and most doctors prefer to prescribe it for women who are breast-feeding. PTU crosses the placenta and goes into breast milk in smaller active amounts than does CMZ.

Are there any side effects of the drugs?

Both drugs can cause minor side effects, such as altered taste sensation or nausea.

The most common significant side effect of both drugs is a rash, which is usually a generalised itchy redness. It affects about five in 100 people who take the drug, and clears up if the drug is stopped. The other drug may then be used if this happens.

The most serious potential side effect of both drugs is bone marrow depression causing a lowering of the white blood cells that normally fight infection, a potentially life-threatening condition called agranulocytosis. This is extremely rare and affects a small number of people usually during the first three months of treatment. The incidence is less than one in 500 and is possibly as low as one in 3000. If you develop a sore throat, mouth ulcers or unexplained fever stop taking the tablets immediately and go to your GP or nearest Accident and Emergency department in order that a full blood count can be carried out. Tell the doctor of the medication you are taking and request a white blood cell count. You should not take another dose until you know the count is normal. Sore throats and mouth ulcers are common and your blood test may well be normal but better safe than sorry.

Very rarely, serious liver injury has been reported in patients 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.

What happens if I am intolerant to both drugs?

This is very rare, but in this situation early radioactive iodine or surgery may be the answer. You will need careful preoperative treatment and close supervision by your specialist.

What happens if my hyperthyroidism returns?

You may be restarted on one of the drugs until your thyroid function returns to normal. It is unlikely though that your thyroid will ever function normally without ongoing treatment. Your doctor may therefore recommend ‘definitive treatment’ to solve the problem permanently. The options are radioactive iodine, which is given in the majority of cases, or surgery. You should discuss the options with your doctor.

Can I take CMZ or PTU long term?

In theory there is no reason why not, provided your thyroid gland remains well controlled. It will initially involve regular clinic visits and blood tests every six to 12 months, under the supervision of a specialist, as doses may continue to need to be adjusted, but once you are stable on a low maintenance dose you may be discharged for follow-up by your GP. You will, however, remain at risk of side effects. Many people feel that radioactive iodine is a more straightforward solution.

Can children take antithyroid drugs?

Yes. This is the usual starting treatment for a child with an over-active thyroid gland. The usual dose used is CMZ 0.5-1mg per kg bodyweight per day or PTU 5-10mg per kg bodyweight per day. Aside from the difference in dose the same considerations apply for children as for adults. PTU is not recommended for children unless they are allergic to CMZ.

Some important points….

  • You will normally be referred to an endocrinologist - a doctor specialising in thyroid and other endocrine disorders
  • It is important to take your tablets every day. Forgetting to take your tablets will affect your blood test results and your health
  • Some medications can affect the blood test results, so it is important to tell your doctor about any other medication you are taking
  • An abnormal blood test result could be due to common illnesses. These sometimes influence the result
  • You must see a doctor immediately if you develop a severe sore throat, mouth ulcers or unexplained fever and ask for a white blood cell count
  • You should see a doctor immediately if you notice yellowing of the eyes or skin, and ask for a liver enzyme test
  • If you are pregnant, or are planning to have a baby, you should tell your doctor and you may need adjustment of your medication and more frequent blood tests
  • You should not be on ‘block and replace’ treatment if you are pregnant or are planning to become pregnant whilst on antithyroid treatment.

It is well recognised that 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.

If you have questions or concerns about your thyroid disorder, you should talk to your doctor or specialist as they will be best placed to advise you. You may of course contact the British Thyroid Foundation for further information and support, or if you have any comments about the information contained in this leaflet.


First issued: February 2008. Revised October 2011. Our literature is reviewed every two years and revised if necessary.