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

The thyroid is a small butterfly-shaped gland in the neck, just in front of the windpipe (trachea). It produces hormones that affect things such as your heart rate and body temperature.

Having too much of these hormones can cause unpleasant and potentially serious problems that may need treatment.

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

Symptoms of an overactive thyroid

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

• nervousness, anxiety and irritability
• mood swings
• difficulty sleeping
• persistent tiredness and weakness
• sensitivity to heat
• swelling in your neck from an enlarged thyroid gland (goitre)
• an irregular and/or unusually fast heart rate (palpitations)
• twitching or trembling
• weight loss

When to see a GP

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

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

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

Treatments for an overactive thyroid

An overactive thyroid is usually treatable.

The main treatments are:

• medicine that stops your thyroid producing too much of the thyroid hormones
• radioiodine treatment – where a type of radiotherapy is used to destroy cells in the 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 usually see a specialist in hormonal conditions (endocrinologist) to discuss which treatment is best for you.

Causes of an overactive thyroid

There are several reasons why your thyroid can become overactive.

These include:

• Graves' disease – a condition where your immune system mistakenly attacks and damages the thyroid (about 3 in every 4 people with an overactive thyroid have Graves' disease)
• lumps (nodules) on the thyroid – this extra thyroid tissue can produce thyroid hormones, causing your levels to be too high
• some medicines such as amiodarone, which can be used to treat an irregular heartbeat (arrhythmia)

Further problems

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

These include:

• eye problems – such as eye irritation, double vision or bulging eyes
• pregnancy complications – such as pre-eclampsia, premature birth or miscarriage
• a thyroid storm – a sudden and life-threatening flare-up of symptoms

Read more about the complications of an overactive thyroid.

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

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

Common symptoms

Symptoms of an overactive thyroid can include:

• nervousness, anxiety and irritability
• hyperactivity – you may find it hard to stay still and have a lot of nervous energy
• mood swings
• difficulty sleeping
• feeling tired all the time
• sensitivity to heat
• muscle weakness
• diarrhoea
• needing to pee more often than usual
• persistent thirst
• itchiness
• loss of interest in sex

Common signs

An overactive thyroid can also cause the following physical signs:

• a swelling in your neck caused by an enlarged thyroid gland (goitre)
• an irregular and/or unusually fast heart rate (palpitations)
• twitching or trembling
• warm skin and excessive sweating
• red palms of your hands
• loose nails
• a raised, itchy rash – known as hives (urticaria)
• patchy hair loss or thinning
• weight loss – often despite an increased appetite
• eye problems, such as redness, dryness or vision problems

When to see a GP

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

These symptoms and signs can have a number of causes. But a blood test can often help to determine whether they're caused by a thyroid problem.

Read more about how an overactive thyroid is diagnosed.

If you have an overactive thyroid (hyperthyroidism), your thyroid gland produces too much of the thyroid hormones.

This results in high levels of the 2 main thyroid hormones, triiodothyronine (also called T3) and thyroxine (also called T4) in your body.

There are a number of conditions that can cause your thyroid to become overactive.

Graves' disease

About 3 in every 4 people with an overactive thyroid gland have a condition called Graves' disease.

Graves' disease is an autoimmune condition where your immune system mistakenly attacks your thyroid which causes it to become overactive.

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

Thyroid nodules

Less commonly, your thyroid can become overactive if lumps (nodules) develop on your thyroid.

Nodules are usually non-cancerous (benign), but they may contain thyroid tissue, which can result in the production of excess thyroid hormones.

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

Medicine

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

This can occasionally happen if you're taking medicine that contains iodine, such as amiodarone, which is sometimes used to control an irregular heartbeat (arrhythmia).

An overactive thyroid that's caused by a medicine will usually improve once you stop taking that medicine, 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 your body – this can happen in early pregnancy, a multiple pregnancy or a molar pregnancy (where a lump of abnormal cells grows in the womb instead of a healthy foetus)
• a pituitary adenoma – a non-cancerous (benign) tumour in the pituitary gland (a gland at the base of the brain that can affect the level of hormones produced by your thyroid)
• thyroiditis – swelling (inflammation) of your thyroid, which can cause extra thyroid hormones to be produced
• thyroid cancer – rarely, a cancerous thyroid tumour can affect the production of thyroid hormones

See a 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

A GP may arrange for you to have a blood test to check your thyroid hormone levels. This is known as a thyroid function test.

The test checks the levels of:

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

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

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

Further tests

If your thyroid hormone levels are high, you may be referred to a specialist for further tests to find out what's causing it.

Further blood tests

You may have another blood test 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, the increase in thyroid hormones is caused by thyroiditis (inflammation of the thyroid).

Thyroid scan

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

You'll be asked to swallow a small amount of a slightly radioactive substance that will be absorbed by your thyroid. It can also be given by injection.

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

An overactive thyroid (hyperthyroidism) is usually treatable.

You'll usually be referred to a specialist in hormonal conditions (endocrinologist) who will plan your treatment.

The main treatments are:

• medicine
• radioactive iodine treatment
• surgery

Medicine

Medicines called thionamides are commonly used to treat an overactive thyroid. They stop your thyroid producing excess hormones.

The main types used are carbimazole and propylthiouracil.

You'll usually need to take the medicine for 1 to 2 months before you notice any benefit. You may also be given another medicine called a beta blocker to help relieve some of your symptoms in the meantime.

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

Side effects

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

• feeling sick
• high temperature
• headaches
• aching joints
• altered taste
• upset stomach
• an itchy rash

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

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

Contact your doctor immediately if you get symptoms of agranulocytosis, such as a high temperature, sore throat, or a persistent cough. They may arrange for a blood test to check your white blood cell count.

Radioactive iodine treatment

Radioactive iodine treatment is a type of radiotherapy is used to destroy the cells in the thyroid gland, 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 iodine and a low dose of radiation, which is absorbed by your thyroid. Most people only need a single treatment.

It can take a few weeks or months for the full benefits to be felt, so you may need to take medicine, such as carbimazole or propylthiouracil, for a short time.

The dose of radiation used during radioactive iodine treatment 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 6 months
• men should not father a child for at least 4 months

Radioactive iodine treatment is not suitable for women who are pregnant or breastfeeding. It's also not suitable if your overactive thyroid is causing severe eye problems.

Surgery

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 because of a large goitre
• you have severe eye problems caused by an overactive thyroid
• you cannot have other, less invasive treatments
• your symptoms return after trying other treatments

Removing all of the thyroid gland is usually recommended because it stops the symptoms of hyperthyroidism coming back.

But you'll need to take medicine for the rest of your life to make up for not having a thyroid gland. These are the same medicines that are used to treat an underactive thyroid.

Several complications can develop if you have an overactive thyroid (hyperthyroidism), particularly if the condition is not treated.

Eye problems

Eye problems, known as thyroid eye disease or Graves' ophthalmopathy, affect around 1 in 4 people with an overactive thyroid caused by Graves' disease.

Problems can include:

• eyes feeling dry and gritty
• sensitivity to light
• watering eyes
• blurred or double vision 
• red eyes
• red, swollen or pulled back eyelids
• bulging eyes

Many people have mild eye symptoms, which get better as their overactive thyroid is treated. But some people may continue to have problems with their eyes.

If Graves’ ophthalmology is particularly severe it can cause vision loss.

If you have eye problems, you may be referred to an eye specialist (ophthalmologist) for treatment, such as eye drops, steroid medicine or possibly surgery.

Underactive thyroid

Radioactive iodine treatment for an overactive thyroid often causes hormone levels to become too low. This is known as an underactive thyroid (hypothyroidism).

Symptoms of an underactive thyroid can include:

• sensitivity to cold
• tiredness
• weight gain
• constipation 
• depression 

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

Pregnancy problems

It's very important that you use contraception if:

• you’re taking carbimazole
• you’ve had radioactive iodine treatment in the last 6 months

Both treatments can harm your unborn baby.

Tell your doctor immediately if you’re planning a pregnancy or become pregnant while taking carbimazole, or after having radioactive iodine treatment within the past 6 months.

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

• pre-eclampsia
• miscarriage
• premature labour and birth (before the 37th week of pregnancy)
• your baby having a low birthweight

Your doctor will want to check whether your condition is under control, and if you’re taking carbimazole they may recommend switching to a treatment that will not affect your baby, such as the medicine propylthiouracil.

Thyroid storm

Rarely, an undiagnosed or poorly controlled overactive thyroid can lead to a serious, life-threatening condition called a thyroid storm.

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

• an infection
• pregnancy
• not taking your medicine 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
• high blood pressure (hypertension)
• 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 to ask for an ambulance immediately.

Other problems

An overactive thyroid can also increase your chances of developing:

• atrial fibrillation – a heart condition that causes an irregular and often abnormally fast heart rate
• weakened bones (osteoporosis) – a condition that makes your bones fragile and more likely to break
• heart failure – the heart is not able to pump blood around the body properly