An overactive thyroid, also known as hyperthyroidism, is where the thyroid gland produces too much of the thyroid hormones.

The thyroid is found at the front of the neck. It produces hormones that affect things such as your heart rate and body temperature.

Extra levels of these hormones can cause unpleasant and potentially serious problems that may require treatment.

An overactive thyroid can affect anyone, but it's about 10 times more common in women than men and it typically starts between 20 and 40 years of age.

This section covers:

Symptoms of an overactive thyroid

An overactive thyroid can cause a wide range of symptoms, including:

Read more about the symptoms of an overactive thyroid.

When to see your GP

See your GP if you have symptoms of an overactive thyroid.

They will ask about your symptoms and can arrange a blood test to check how well your thyroid is working if they think you might have a thyroid problem.

If the blood test shows that you have an overactive thyroid, you may be referred for further tests to identify the cause.

Read more about how an overactive thyroid is diagnosed.

Treatments for an overactive thyroid

An overactive thyroid is usually treatable.

The main treatments are:

  • medication that stops your thyroid producing too much of the thyroid hormones
  • radioiodine treatment – where radiation is used to damage your thyroid, reducing its ability to produce thyroid hormones
  • surgery to remove some or all of your thyroid, so that it no longer produces thyroid hormones

Each of these treatments has benefits and drawbacks. You'll normally see an endocrinologist (specialist in hormone conditions) to discuss which is best for you.

Read more about how an overactive thyroid is treated.

Causes of an overactive thyroid

There are a number of reasons why the thyroid can become overactive.

These include:

  • Graves' disease – where the body's immune system mistakenly attacks and damages the thyroid
  • lumps (nodules) on the thyroid – this results in extra thyroid tissue, which can mean extra thyroid hormones are produced
  • certain medications – such as amiodarone, a medication for an irregular heartbeat (arrhythmia)

About three in every four cases are because of Graves' disease.

Read more about the causes of an overactive thyroid.

Further problems

An overactive thyroid, particularly if it's not treated or well controlled, can sometimes lead to further problems.

These include:

Read more about the complications of an overactive thyroid.

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An overactive thyroid (hyperthyroidism) can cause a wide range of possible symptoms, although it's unlikely you'll experience all of them.

The symptoms may develop gradually or suddenly. For some people they're relatively mild, while for others they can be severe and significantly affect their life.

Common symptoms

Symptoms of an overactive thyroid can include:

Common signs

An overactive thyroid can also cause the following physical signs:

When to see your GP

See your GP if you have symptoms of an overactive thyroid. It might be useful to make a list of all your symptoms and show it to your GP.

These symptoms can have a number of causes. But a simple blood test can often help determine whether they're caused by a problem with your thyroid.

Read more about how an overactive thyroid is diagnosed.

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An overactive thyroid (hyperthyroidism) occurs when your thyroid gland produces too much of the thyroid hormones. This results in a high level of thyroid hormones called triiodothyronine (also called "T3") and thyroxine (also called "T4") in your body.

The thyroid can become overactive for a number of reasons. Some of the main causes are described below.

Graves' disease

In about three in every four cases, an overactive thyroid is caused by a condition called Graves' disease.

This is an autoimmune condition, which means the immune system mistakenly attacks the body. In Graves' disease, it attacks the thyroid and causes it to become overactive.

The cause of Graves' disease is unknown, but it mostly affects young or middle-aged women and it often runs in families. Smoking can also increase your risk of getting it.

Thyroid nodules

Less commonly, an overactive thyroid can occur if lumps called nodules develop on the thyroid. These nodules are usually non-cancerous (benign), but they contain additional thyroid tissue, which can result in the production of excess thyroid hormones.

It's not known why some people develop thyroid nodules, but they most often affect people over 60 years of age.


An increased level of iodine in the body can cause the thyroid to produce excess thyroid hormones.

This can occasionally occur if you're taking medication that contains iodine, such as amiodarone – a medicine sometimes used to control an irregular heartbeat (arrhythmia).

An overactive thyroid caused by medication will usually improve once the medication is stopped, although it may take several months for your thyroid hormone levels to return to normal.

Other causes

Other possible causes of an overactive thyroid include:

  • high levels of a substance called human chorionic gonadotrophin in the body – this can occur in early pregnancy, a multiple pregnancy or a molar pregnancy (where tissue remains in the womb after an unsuccessful pregnancy)
  • a pituitary adenoma – a non-cancerous (benign) tumour of the pituitary gland (a gland located at the base of the brain that can affect the level of hormones produced by the thyroid)
  • thyroiditis – inflammation of the thyroid, which can result in extra thyroid hormones being produced
  • thyroid cancer – rarely, a cancerous thyroid tumour can affect the production of thyroid hormones
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See your GP if you think you may have an overactive thyroid (hyperthyroidism).

A diagnosis will be based on your symptoms and the results of a blood test that assesses how well your thyroid is working.

Thyroid function blood test

Your GP may take a sample of blood and send it to a laboratory to check your hormone levels. This is known as a thyroid function test.

It's used to check the levels of:

  • thyroid-stimulating hormone (TSH) – a hormone produced by the pituitary gland (a gland at the base of the brain) that controls the production of thyroid hormones
  • triiodothyronine (T3) – one of the main thyroid hormones
  • thyroxine (T4) – another of the main thyroid hormones

Your levels will be compared to what's normal for a healthy person. A low level of TSH and high levels of T3 and/or T4 usually means you have an overactive thyroid.

What are considered normal levels varies depending on things such as your age and the exact testing technique used by the laboratory.

Further tests

If you have high thyroid hormone levels, you may be referred to a specialist for the tests below to find out what's causing it.

Further blood tests

Another blood test may be carried out to look for anti-thyroid antibodies.

These are usually found if you have Graves' disease, a common cause of an overactive thyroid.

A blood test called erythrocyte sedimentation rate (ESR) may also be done to check for inflammation in your body.

If there are signs of inflammation, it may mean the increase in thyroid hormones is caused by thyroiditis (inflammation of the thyroid).

Thyroid scan

A thyroid scan may be done to look for problems such as lumps (nodules) on your thyroid.

For the test, you'll be asked to swallow or have an injection of a small amount of a slightly radioactive substance that will be absorbed by your thyroid.

A scan is then carried out to see how much of the substance has been absorbed and to examine the size and shape of your thyroid.

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An overactive thyroid (hyperthyroidism) is usually treatable.

You'll normally be referred to an endocrinologist (specialist in hormone conditions) to plan your treatment.

The main treatments are:

These are outlined below.


Medicines called thionamides are a common treatment for an overactive thyroid. These stop your thyroid producing excess hormones.

The main types used are carbimazole and propylthiouracil.

You'll usually need to take the medicine for a month or two before you notice any benefit. You may be given another medication called a beta-blocker to quickly relieve your symptoms in the meantime.

Once your thyroid hormone level is under control, your dose may be gradually reduced and then stopped. But some people need to continue taking medication for several years or possibly for life.

Side effects

During the first couple of months, some people experience the following side effects:

These should pass as your body gets used to the medication.

A less common but more serious side effect is a sudden drop in your white blood cell level (agranulocytosis), which can mean you're very vulnerable to infections.

Contact your doctor immediately if you get symptoms of agranulocytosis, such as a fever, sore throat or persistent cough so a blood test can be carried out to check your white blood cell level.

Radioiodine treatment

Radioiodine treatment is where radiation is used to damage your thyroid, reducing the amount of hormones it can produce. It's a highly effective treatment that can cure an overactive thyroid.

You're given a drink or capsule that contains a low dose of radiation, which is then absorbed by your thyroid. Most people only require a single treatment.

It can take a few weeks or months for the full benefits to be felt, so you may need to take one of the medications mentioned above for a short time.

The dose of radiation you're given is very low, but there are some precautions you'll need to take after treatment:

  • avoid prolonged close contact with children and pregnant women for a few days or weeks
  • women should avoid getting pregnant for at least six months
  • men shouldn't father a child for at least four months

Radioiodine treatment isn't suitable if you're pregnant or breastfeeding. It's also not suitable if your overactive thyroid is causing severe eye problems.


Occasionally, surgery to remove all or part of your thyroid may be recommended.

This may be the best option if:

  • your thyroid gland is severely swollen (a large goitre)
  • you have severe eye problems resulting from an overactive thyroid
  • you can't have the other treatments mentioned above
  • your symptoms come back after trying the treatments mentioned above

Removing the entire thyroid gland is normally recommended, as this cures an overactive thyroid and means there's no chance of the symptoms coming back.

But as a result, you'll need to take medication for the rest of your life to make up for not having a thyroid – these are the same medications used to treat an underactive thyroid.

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Several complications can occur with an overactive thyroid (hyperthyroidism), particularly if the condition isn't treated.

Eye problems

Eye problems, known as thyroid eye disease or Graves' ophthalmopathy, affect around one in three people with an overactive thyroid caused by Graves' disease.

These can include:

Many cases are mild and get better as your overactive thyroid is treated, but in around one in every 20 to 30 cases there's a risk of vision loss.

If you experience eye problems, you will probably be referred to an eye specialist (ophthalmologist) for treatment, such as eye drops, steroid medication or possibly surgery.

Read more about how thyroid eye disease is treated.

Underactive thyroid

Treatment for an overactive thyroid often results in hormone levels that are too low – known as an underactive thyroid (hypothyroidism).

Symptoms of an underactive thyroid can include:

An underactive thyroid is sometimes only temporary, but often it's permanent and requires long-term treatment with thyroid hormone medication.

Read more about the how an underactive thyroid is treated.

Pregnancy problems

If you have an overactive thyroid during pregnancy and your condition isn't well controlled, it can increase the risk of:

Tell your doctor if you're planning a pregnancy or think you might be pregnant.

They will want to check whether your condition is under control and they may recommend switching to a treatment that won't affect the baby, such as the medication propylthiouracil.

If you're not planning a pregnancy, it's important to use contraception because some treatments for an overactive thyroid can harm an unborn baby.

Thyroid storm

In rare cases, an undiagnosed or poorly controlled overactive thyroid can lead to a serious, life-threatening reaction called a thyroid storm.

This is a sudden flare-up of symptoms that can be triggered by:

  • an infection
  • pregnancy
  • not taking your medication correctly
  • damage to the thyroid gland, such as a punch to the throat

Symptoms of a thyroid storm include:

  • a rapid heartbeat
  • a high temperature (fever) over 38C (100.4F)
  • diarrhoea and vomiting
  • yellowing of the skin and eyes (jaundice)
  • severe agitation and confusion
  • loss of consciousness

A thyroid storm is a medical emergency. If you think you or someone in your care is experiencing it, call 999 for an ambulance immediately.

Other problems

An overactive thyroid can also increase your chances of developing:

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Selected links

NHS Direct Wales links

Underactive thyroid


External link

British Thyroid Foundation

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The information on this page has been adapted by NHS Wales from original content supplied by NHS Choices.
Last Updated: 12/10/2016 08:55:18