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Antithyroid medicines

Antithyroid medicines are used to treat an overactive thyroid gland (hyperthyroidism). Carbimazole is the most commonly used medicine. 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 antithyroid drug therapy, 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. An overactive thyroid is potentially serious and you should see a qualified doctor before trying herbal or homeopathic remedies.

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What are antithyroid medicines?

Antithyroid medicines are used to treat an overactive thyroid gland (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 overactive thyroid disorder.

  • 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.

See the separate leaflet called Overactive Thyroid Gland (Hyperthyroidism).

How do antithyroid drugs work?

Thyroxine (also known as T4) is a body chemical (hormone) made by the thyroid gland. It is carried around 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.

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Which is the best treatment for an overactive thyroid gland?

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. Propylthiouracil is also used for an overactive thyroid gland in pregnancy. 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.

Option 1

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.

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.

Option 2

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.

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How quickly does antithyroid medication work?

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.

How long is treatment needed for an overactive thyroid gland?

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.

Will hyperthyroidism return after treatment with antithyroid medicines?

As mentioned before, having an overactive thyroid gland (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 an overactive thyroid gland (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 the separate leaflet called Overactive Thyroid Gland (Hyperthyroidism) for details of these other treatment options.

What are the possible side-effects of antithyroid medication?

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 bone marrow which produces red blood cells, white blood cells and platelets. Antithyroid drug therapy can reduce the numbers of these cells being produced. The lower number of red blood cells causes anaemia. The effect on white blood cells (or neutrophils) is called agranulocytosis. The drastically reduced number of white blood cells in your body may mean you are less able to fight off infection. The reduced number of platelets may make you more likely to bleed. Therefore, if you experience any of these whilst on the medicine, you must stop taking it and report this to your doctor immediately, if you develop:

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 antithyroid medicines 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. 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.

Are there homeopathy treatments for overactive thyroid instead of thyroid medicine?

When you look up this condition on the internet, there will be some kind of 'natural remedy' that promises a great cure without all those 'nasty medicines' that doctors prescribe.

Be careful: an overactive thyroid gland is a potentially serious condition, particularly for your heart. It's best to get an opinion from a qualified doctor (MBChB or MBBS), preferably one who is impartial and paid by a state-funded system like the NHS, before trying any homeopathic treatments.

Can I use thyroid medicines for weight loss?

When someone has an underactive thyroid gland, they are given a medicine to boost their levels of thyroxine. This medicine is called levothyroxine. Sometimes this can help the person lose weight, if their underactive thyroid gland has caused them to put on weight. See the separate leaflet called Underactive Thyroid Gland (Hypothyroidism).

Unfortunately, as you can imagine, some unscrupulous people get hold of levothyroxine and then try to sell it as a 'weight loss' cure.

Be careful: it's true that taking lots of levothyroxine could make you lose weight but it will also be very harmful to your body. It will turn you into having an overactive thyroid gland (hyperthyroidism) with all of the bad effects of that condition that have been discussed in this leaflet.

The best and healthiest way to lose weight is to eat healthily and be active. See the separate leaflet called Weight Loss (Weight Reduction) for help on losing weight.

Further reading and references

  • Moleti M, Di Mauro M, Sturniolo G, et al; Hyperthyroidism in the pregnant woman: Maternal and fetal aspects. J Clin Transl Endocrinol. 2019 Apr 12;16:100190. doi: 10.1016/j.jcte.2019.100190. eCollection 2019 Jun.
  • Hyperthyroidism; NICE CKS, January 2021 (UK access only)
  • Kahaly GJ, Bartalena L, Hegedus L, et al; 2018 European Thyroid Association Guideline for the Management of Graves' Hyperthyroidism. Eur Thyroid J. 2018 Aug;7(4):167-186. doi: 10.1159/000490384. Epub 2018 Jul 25.
  • Abbara A, Clarke SA, Brewster R, et al; Pharmacodynamic Response to Anti-thyroid Drugs in Graves' Hyperthyroidism. Front Endocrinol (Lausanne). 2020 May 12;11:286. doi: 10.3389/fendo.2020.00286. eCollection 2020.

Article history

The information on this page is written and peer reviewed by qualified clinicians.

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