Page last reviewed: 13/07/2011

Overactive thyroid, also known as hyperthyroidism or thyrotoxicosis, is a condition that occurs when there is too much thyroid hormone in the body. The condition is more common in women than men.

The role of the thyroid gland

The thyroid gland is found in the neck. It produces hormones that are released into the bloodstream to control the body's growth and metabolism. These hormones are called thyroxine and triiodothyronine. They affect processes such as heart rate and body temperature, and help convert food into energy to keep the body going.

Normally, the levels of thyroxine and triiodothyronine in the blood are carefully controlled so that these processes happen at a stable rate.

However, in hyperthyroidism, the thyroid gland produces an excess amount of thyroxine or triiodothyronine, which speeds up the body's metabolism. This causes symptoms such as:

  • hyperactivity
  • an increased appetite
  • sudden weight loss or gain

Overt and subclinical hyperthyroidism

There are several causes of an overactive thyroid gland. The most common is Graves' disease, in which the body's immune system targets the thyroid gland and causes it to produce too much thyroid hormone.

Once diagnosed, overactive thyroid can be classed as either:

  • overt, which means that it causes symptoms
  • subclinical, when the levels of thyroid hormone are still within normal limits


Overactive thyroid can be effectively treated using medicines to slow the production of hormones from the thyroid gland, as well as radiotherapy and in some cases surgery.

However, if it is not successfully controlled, overactive thyroid can cause other health problems such as irregular heart rhythms and osteoporosis (weak, brittle bones).

If it is not treated, overactive thyroid can be fatal.

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Page last reviewed: 13/07/2011

Overactive thyroid (hyperthyroidism) has many symptoms and signs, although it is very unlikely you would develop all of them.

Symptoms of hyperthyroidism

If you have hyperthyroidism, you may have some of the following symptoms:

  • difficulty breathing
  • hyperactivity
  • mood swings, irritability and nervousness
  • difficulty sleeping
  • difficulty concentrating
  • fatigue (extreme tiredness)
  • muscle weakness
  • needing to pass stools (faeces) or urine frequently
  • diarrhoea or steatorrhoea (excess fat in your stools)
  • sensitivity to heat and excess sweating
  • increased appetite
  • sudden weight loss or gain
  • very infrequent or light periods, or periods stopping altogether
  • infertility
  • loss of interest in sex

If you have diabetes, your diabetic symptoms, such as extreme thirst and tiredness, may be made worse by hyperthyroidism.

Signs of hyperthyroidism

If you have hyperthyroidism, you may notice some of the following physical signs:

  • a swelling in your neck caused by an enlarged thyroid gland (goitre)
  • uncoordinated rhythm between your heartbeat and pulse
  • a rapid resting heart rate
  • a tremor (trembling or shaking)
  • warm, moist skin
  • redness on the palms of your hands
  • loosening of your nails in their nail beds
  • itchy skin with raised itchy swellings (urticaria)
  • patchy hair loss (alopecia)
  • twitching in your face and limbs

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Page last reviewed: 13/07/2011

Overactive thyroid (hyperthyroidism) occurs when your thyroid gland produces too much of the thyroid hormones thyroxine or triiodothyronine.

Overproduction of thyroid hormones can be caused by a number of conditions, which are outlined below.

Graves' disease

Graves' disease is the most common cause of overactive thyroid. It can run in families and can occur at any age, although it is most common in women aged 20-40 years old. You are more likely to develop Graves' disease if you smoke.

Graves' disease is an autoimmune condition. This means the immune system mistakes a healthy substance in the body for a toxic substance and attacks it. Normally, your immune system makes antibodies (proteins) to attack bacteria and fight infection. However, in autoimmune conditions, the antibodies attack healthy tissues instead. It is not known what triggers the immune system to do this.

If you have Graves' disease, your eyes may also be affected, causing discomfort and double vision. This is known as Graves' ophthalmopathy. You may find that your eyes bulge out, or appear more prominent.

It is not fully understood why the eyes are affected by Graves' disease, but it is thought that antibodies from your immune system may also affect the tissues around the eye.

Thyroid nodules

It is possible for lumps to develop in your thyroid gland. These are known as nodules. It is not known why nodules develop, but they are usually benign (non-cancerous).

However, the nodules can contain abnormal thyroid tissue, which can affect the normal production of thyroxine or triiodothyronine, causing overactive thyroid. Nodules that contain abnormal thyroid tissue are described as toxic.

Toxic multinodular goitre is the second most common cause of hyperthyroidism after Graves' disease, and occurs when there are two or more nodules in your thyroid gland.

However, if you only have a single nodule in your thyroid gland, it is known as a toxic thyroid nodule or adenoma. Toxic thyroid nodules account for about 5% of cases of hyperthyroidism.


Occasionally a viral infection can affect the thyroid gland and cause inflammation within the thyroid called a thyroiditis where the gland can become painful and sore to touch.  Thyroiditis is associated with transient over-activity which can last for a few weeks before the thyroid function swings and becomes underactive (hypothyroidism) before finally it returns to normal thyroid function.   Thyroiditis is sometimes also seen after delivery of a baby called post-partum thyroiditis.  

Iodine supplements

Iodine contained in the food you eat is used by your thyroid gland to produce the thyroid hormones thyroxine and triiodothyronine. However, taking additional iodine in supplements can cause your thyroid gland to produce too much thyroxine or triiodothyronine.

The type of overactive thyroid that can result from this is known as iodine-induced hyperthyroidism, sometimes referred to as Jod-Basedow phenomenon. It usually only occurs if you already have non-toxic nodules in your thyroid gland.


Amiodarone is a type of medication known as an anti-arrhythmic, which helps to control an irregular heartbeat (atrial fibrillation). If you have non-toxic nodules in your thyroid gland, taking amiodarone can cause hyperthyroidism because it contains iodine. This type of hyperthyroidism is called amiodarone-induced hyperthyroidism.

Antibodies and immunoglobulins are proteins in the blood. They are produced by the immune system to fight against bacteria, viruses and disease.
Inflammation is the body's response to infection, irritation or injury, which causes redness, swelling, pain and sometimes a feeling of heat in the affected area.
Immune system
The immune system is the body's defence system, which helps protect it from disease, bacteria and viruses.
A nodule is a small growth or lump of tissue.
The thyroid is a jointed piece of cartilage that encloses the vocal cords and forms the 'Adam's apple' in men.
Thyroid gland
The thyroid gland in the throat makes hormones to help control growth and metabolism (the process that turns the food we eat into energy).
Body tissue is made up of groups of cells that perform a specific job, such as protecting the body against infection, producing movement or storing fat.  

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Page last reviewed: 13/07/2011

See your GP if you think you may have overactive thyroid (hyperthyroidism).

A diagnosis of overactive thyroid will be based on your symptoms and the results of a blood test, which assesses how well your thyroid gland is working. This blood test is known as a thyroid function test.

Thyroid function test

Your GP will take a sample of your blood and test it for levels of:

  • thyroid-stimulating hormone (TSH)
  • thyroxine and triiodothyronine (the thyroid hormones)

TSH is made in the pituitary gland in your brain and controls the production of thyroxine and triiodothyronine.

  • When the level of thyroxine and triiodothyronine in your blood are normal, your pituitary gland stops releasing TSH.
  • When the level of thyroxine or triiodothyronine drops, the pituitary gland produces more TSH to boost the level.

If you have overactive thyroid, the thyroid function test will show that the levels of TSH in your blood are consistently lower than normal. Low levels of TSH mean your thyroid gland is overactive and is likely to be making too many thyroid hormones. This is the first part of the thyroid function test.

Your GP will then test your blood for levels of thyroxine and triiodothyronine. If you have overactive thyroid, you will have higher than normal levels of both of these hormones, which causes the symptoms of overactive thyroid.

Types of hyperthyroidism

If your thyroid function test confirms a diagnosis of hyperthyroidism, your GP should be able to tell you whether you have overt (fully developed) overactive thyroid or subclinical overactive thyroid (where your symptoms are mild or absent).

If you have overt overactive thyroid, your blood may have low levels of TSH and high levels of thyroxine or triiodothyronine. Your symptoms are likely to be moderate or severe.

If you have subclinical hyperthyroidism, your blood may have low levels of TSH but normal levels of thyroxine and triiodothyronine. You may have no symptoms at all.

It is possible for subclinical overactive thyroid to develop into overt overactive thyroid.

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Treatment for overactive thyroid (hyperthyroidism) aims to return the level of thyroid hormones in your blood to normal. You may also need treatment for any associated conditions, such as swelling of your thyroid gland (goitre).

Treating subclinical overactive thyroid

If you are diagnosed with subclinical overactive thyroid, you may not need any treatment for your condition.

In most cases, the reduced level of thyroid-stimulating hormone (TSH) in your blood will eventually return to normal within a couple of months without the need for treatment, and your subclinical hyperthyroidism will resolve by itself.

However, as subclinical overactive thyroid can progress to overt (fully developed) overactive thyroid, you will need to return to your GP two to three months after your diagnosis for a thyroid function test, so your condition can be monitored.

If you still have lower than normal levels of TSH in your blood after two to three months, without raised levels of thyroid hormones, you will need to return to your GP regularly for further testing.

This may be once every three to six months, or more often if you are elderly or you have an underlying condition that affects the heart, arteries and veins, such as coronary heart disease or stroke.

Your GP may also suggest further assessments to determine the cause of your subclinical overactive thyroid. If your GP is concerned that your condition has not resolved, they may refer you for specialist treatment.

Treating overt overactive thyroid

You will be diagnosed with overt overactive thyroid if the thyroid function test finds that your levels of TSH are low and your levels of thyroid hormone (thyroxine or triiodothyronine) are high.

In this case, your GP will refer you for specialist treatment. You may also be referred for specialist treatment if:

  • you have Graves' ophthalmopathy (eye problems such as double vision and prominent or bulging eyes caused by Graves' disease.
  • you are pregnant, planning a pregnancy or have recently given birth and you have existing or newly diagnosed overactive thyroid
  • you have overactive thyroid that is caused by a secondary substance, such as amiodarone
  • you have an underlying condition, such as coronary heart disease

You may need to receive emergency treatment in hospital if you have any of the symptoms of a thyroid storm . This is a rare but serious reaction that can occur as a result of undiagnosed or poorly controlled overactive thyroid.

Your specialist will determine the best method of treatment based on your symptoms and the amount of extra thyroid hormone in your blood. The methods of treating overactive thyroid are outlined below.

Thionamides (carbimazole and propylthiouracil)

Thionamides are a common treatment for overactive thyroid. They are a type of medication that stops your thyroid gland producing excess amounts of thyroxine or triiodothyronine.

As thionamides affect the production of thyroid hormone rather than their current levels, you will need to take them for several weeks before you notice an improvement.

You will probably need to take carbimazole or propylthiouracil for four to eight weeks before your thyroid gland is under control.

Once the production of thyroid hormone from your thyroid gland is under control, your specialist may gradually reduce the dosage of medication.

If your condition is not under control, you may need to continue taking carbimazole or propylthiouracil for a long time. You may also have to take this medication in the long term if your overall health prevents you from using other forms of treatment.

In rare cases, you may experience several side effects from taking carbimazole or propylthiouracil, although these are usually mild. Side effects may include:

  • mild skin rash
  • pain in your joints
  • nausea
  • itchy skin

In very rare cases, carbimazole or propylthiouracil can cause a serious blood condition known as agranulocytosis (a sudden drop in white blood cells). This usually occurs in the first three months of treatment.

Seek urgent medical attention if you are taking a thionamide and you develop:

  • fever
  • sore throat
  • mouth ulcers
  • other signs of infection


Beta-blockers, such as propranolol or atenolol, can relieve some of the symptoms of overactive thyroid, including tremor (shaking and trembling), rapid heartbeat and hyperactivity.

Your specialist may prescribe you a beta-blocker to relieve your symptoms while you are undergoing a diagnostic assessment, or until your thyroid gland is brought under control by treatment with a thionamide. However, beta-blockers are not suitable if you have asthma.

Beta-blockers can sometimes cause side effects, including:

  • nausea
  • fatigue (extreme tiredness)
  • cold hands and feet
  • trouble sleeping, sometimes with nightmares

Radioiodine treatment

Radioiodine treatment is used to treat most types of overactive thyroid. It contains radioactive iodine, which builds up in your thyroid gland and shrinks it, reducing the amount of thyroid hormone that it can make.

If you have radioiodine treatment, you will be given either a drink or a capsule of radioiodine to swallow. The dose of radioactivity contained in the radioiodine is very low and is not harmful.

Radioiodine treatment is not suitable if you are pregnant or breastfeeding, and may not be suitable if you have eye problems, such as double vision or prominent (bulging) eyes.

Women should avoid getting pregnant for at least six months after having radioiodine treatment. Men should not father a child for at least six months after having radioiodine treatment.

Surgery (thyroidectomy)

Surgery to remove all or part of the thyroid gland is known as a total or near-total thyroidectomy. It is a permanent cure for recurrent overactive thyroid.

Your specialist may recommend surgery if your thyroid gland is severely swollen (a large goitre) and is causing problems in your neck.

A thyroidectomy may also be suggested if:

  • you have toxic multinodular goitre or a toxic thyroid nodule (adenoma) or Graves' disease
  • you also have cancer of the thyroid

The goal of surgery is to remove the thyroid gland safely and control the overactivity. After surgery you may get underactive thyroid (hypothyroidism). This happens when not enough thyroid hormones are produced.

However, if underactive thyroid occurs, thyroid hormone tablets can be taken to keep your thyroid levels normal.

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Page last reviewed: 13/07/2011

Several complications can occur if you have overactive thyroid (hyperthyroidism). These are outlined below and are most likely to occur if the condition is not treated.

Graves' ophthalmopathy

If you have Graves' disease, you may have problems with your eyes. This is known as Graves' ophthalmopathy.

Problems may include:

  • discomfort in your eyes
  • problems with vision
  • prominent (bulging) eyes

In mild cases, the symptoms of Graves' ophthalmopathy can be relieved using eye drops. You can prevent dust particles getting into your eyes by wearing sunglasses and eye protectors while you are sleeping. More severe forms of the condition require specialist treatment.

Thyroid storm

Overactive thyroid that is undiagnosed or poorly controlled can lead to a rare but serious reaction called a thyroid storm.

A thyroid storm can be triggered by an infection, injury or trauma. The condition can also occur in pregnant women with undiagnosed or poorly controlled hyperthyroidism, and can be triggered by childbirth or surgery.

The symptoms of a thyroid storm include:

  • a very rapid heartbeat (over 140 beats a minute)
  • fever (a temperature higher than 38°C/100.4°F)
  • dehydration, with diarrhoea and vomiting
  • jaundice (a yellow tinge to your skin)
  • severe agitation and confusion
  • hallucinations (seeing or hearing things that are not real)

A thyroid storm requires emergency medical treatment in hospital.

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Pregnancy and overactive thyroid

Women with overactive thyroid who become pregnant are more at risk of developing complications during pregnancy and birth, such as miscarriage and eclampsia (seizures during pregnancy).

They are also more at risk of going into labour prematurely and having a baby with a low birth weight.

If you are pregnant and have an overactive thyroid, you should be referred for specialist treatment so your condition can be managed using medications that will not affect your baby. Your condition will also need to be carefully monitored throughout your pregnancy.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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