Revised 2018

Your guide to antithyroid drugs

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Content overview

What is hyperthyroidism?

What drugs are used to treat hyperthyroidism?

What is the course of treatment?

Who is more likely to relapse?

Does it matter which drug I am put on?

Are there any side effects of the drugs?

What happens if I am intolerant to both drugs?

What happens if my hyperthyroidism returns?

Can I take CMZ or PTU long term?

Can children take antithyroid drugs?

Some important points

What is hyperthyroidism?

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

What drugs are used to treat hyperthyroidism?

There are two antithyroid drugs commonly used in the UK: Carbimazole (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). The first choice drug is CMZ. If this is not tolerated or there are plans for pregnancy PTU is used.

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 overactive 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 (or 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 50-150mcg 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 underactive thyroid.

You will usually continue on antithyroid drugs alone for up to 18 months, or on block and replace therapy for six to 12 months. If you have Graves’ disease there is about a 30-50% 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 overactive 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.

You may have got used to increased food intake without weight gain during the period of thyroid over-activity (increased metabolism). Once the thyroid function and metabolism is normalised by any of the forms of treatment, you may have to reduce your food intake to avoid undesirable weight gain.

Who is more likely to relapse?

There is more chance of continuing problems if:

• there have been problems controlling your hyperthyroidism with drugs
• you require high doses of treatment
• you have a very large thyroid gland
• you have thyroid eye disease
• you have high TSH receptor antibodies

Smokers are up to three times more likely to relapse than non-smokers. Women and those over the age of 40 seem less likely to relapse after a course of treatment.

The above factors may be used to consider whether to move earlier to definitive treatment with radioiodine or surgery, particularly in women who are planning to start a family.

Does it matter which drug I am put on?

Most doctors prescribe CMZ in the first instance.

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

PTU currently 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 usually recommended during the first three months of pregnancy, and it may be considered as second line in women who require large doses of treatment when they are breast-feeding. However both PTU and CMZ are linked with birth defects, though PTU effects are usually milder. Women who have not yet started a family may wish to discuss radioiodine as a treatment option to reduce exposure during pregnancy and breastfeeding.

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 certainly less than one in 500 and is possibly as low as one in 3,000. 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 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, including children, 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.

Due to the unpredictable and rare nature of these side effects – agranulocytosis and serious liver injury - regular monitoring of blood count or liver functions is not recommended, though some doctors may do a baseline test which may help them monitor future changes.

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. 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 as there is an increased risk of liver damage.

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 all other medication – even if it is over the counter or non-prescription - you are taking
• An abnormal blood test result could be due to common illnesses. These sometimes influence the result
• If you develop a sore throat, mouth ulcers or unexplained fever, you must see a doctor immediately and ask for a white blood cell count
• If you notice yellowing of the eyes or skin you should see a doctor and ask for a liver enzyme test
• If you are pregnant, or are planning to have a baby, you should tell your doctor as you may need to adjust your medication and to have more frequent blood tests
• You should not be on ‘block and replace’ treatment if you are pregnant or are planning to become pregnant

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 also contact the British Thyroid Foundation for further information and support, or if you have any comments about the information contained in this leaflet.

Find more resources, including patient stories, films and details of our support network here 

 Living with hyperthyroidism 

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The British Thyroid Foundation

www.btf-thyroid.org
The British Thyroid Foundation is a registered charity: England and Wales No 1006391, Scotland SC046037

Endorsed by:

The British Thyroid Association - medical professionals encouraging the highest standards in patient care and research
www.british-thyroid-association.org

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)
www.baets.org.uk

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

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