Hyperthyroidism, also known as thyrotoxicosis or overactive thyroid, 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 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 to turn 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 energy consumption functions. As a result, the metabolism is accelerated. This causes symptoms such as:
There are several causes of hyperthyroidism, although 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, hyperthyroidism can be classed as either:
If it isn't successfully controlled, hyperthyroidism can cause several other health problems, such as heart dysrhythmia (irregular heart rhythms) and osteoporosis (weak, brittle bones).
If left untreated hyperthyroidism can be fatal. However, the condition can be treated effectively using medicines to slow the production of hormones from the thyroid gland, as well as radiotherapy and, in some cases, surgery.
Hyperthyroidism occurs when your thyroid gland produces too much of one of the thyroid hormones – thyroxine and triiodothyronine. Excessive production of thyroid hormones can be caused by a number of conditions which are outlined below.
Graves' disease is the most common cause of hyperthyroidism. It can run in families and can occur at any age, although it is most common in women between 20-40 years of age. You are also more likely to develop Graves' disease if you smoke.
Graves' disease is an autoimmune condition, which means that it occurs when your immune system mistakes a healthy substance in your body for a toxic substance and attacks it. Normally, your immune system makes antibodies which attack bacteria and fight infection. However, in autoimmune conditions, the antibodies attack your body's 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' opthalmopathy. You may find that your eyes 'stand 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.
It is possible for lumps to develop in your thyroid gland, which are known as nodules. It is not known why these 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 hyperthyroidism. 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.
The iodine that is contained in the food that 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 hyperthyroidism which can result from this is known as iodine-induced hyperthyroidism, which is 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.
In rare cases, you may develop hyperthyroidism as a result of thyroid cancer that starts in your thyroid follicles. This can occur if the cancer cells in your thyroid gland begin to produce thyroxine, or triiodothyronine, on its own.
You should see your GP if you think that you may have hyperthyroidism. If you have hyperthyroidism, your diagnosis will be based on your symptoms and the results of a blood test which can assess how well your thyroid gland is working. This blood test is known as a thyroid function test.
A thyroid function test assesses your blood in two different ways to confirm a diagnosis of hyperthyroidism. Your GP will take a sample of your blood and test it for levels of thyroid-stimulating hormone (TSH) and levels of the thyroid hormones, thyroxine and triiodothyronine.
TSH is made in the pituitary gland in your brain and controls the production of thyroxine and triiodothyronine. When the levels 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 levels.
If you have hyperthyroidism, the thyroid function test will show that the levels of TSH in your blood are consistently lower than normal. Low levels of TSH mean that 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 hyperthyroidism, you will have higher than normal levels of both of these hormones, which causes the symptoms 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) hyperthyroidism, or subclinical hyperthyroidism (where your symptoms are mild, or absent).
If you have overt hyperthyroidism, your blood may have low levels of TSH, and high levels of thyroxine or triiodothyronine. Your symptoms of hyperthyroidism 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 hyperthyroidism to develop into overt hyperthyroidism.
Hyperthyroidism has many symptoms, although it is very unlikely that you would ever develop all of them. There are also several physical signs which you may notice if you have hyperthyroidism.
Symptoms of hyperthyroidism
If you have hyperthyroidism, you may have some of the following symptoms:
If you have diabetes, you may also find that your diabetic symptoms, such as extreme thirst and tiredness, are made worse by hyperthyroidism.
Signs of hyperthyroidism
If you have hyperthyroidism, you may notice some of the following physical signs:
In rare cases, hyperthyroidism that is undiagnosed or poorly controlled can cause a serious reaction known as a thyroid storm. A thyroid storm can occur as a result of an infection, injury, or trauma, such as a stroke. 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 thyroid storm requires immediate treatment and, left untreated, it can cause a coma. Seek immediate medical help if you think that you, or someone else, is displaying the symptoms of thyroid storm.
Treatment for 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).
If you are diagnosed with subclinical hyperthyroidism, 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 it is possible for subclinical hyperthyroidism to progress into overt (fully developed) hyperthyroidism, you will need to return to your GP 1-2 months after your diagnosis for a thyroid function test so that your condition can be monitored.
If, after 1-2 months, you still have lower than normal levels of TSH in your blood, without raised levels of thyroid hormones, you will need to return to your GP regularly for further testing. This may be once every 3-6 months, or more often if you are elderly, or you have an underlying vascular disease (conditions that affect 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 hyperthyroidism. If your GP is concerned that your condition has not resolved, they may refer you for specialist treatment.
You will be diagnosed with overt hyperthyroidism if the thyroid function test finds that your levels of TSH are low, and your levels of thyroid hormone (thyroxine or triiodothyronine) are high.
If you are diagnosed with overt (fully developed) hyperthyroidism, your GP will refer you for specialist treatment. You may also be referred for specialist treatment if:
You may need to receive emergency treatment in hospital if you have any of the symptoms of a thyroid storm (see 'symptoms' section), which is a rare, but serious reaction that can occur as a result of undiagnosed or poorly controlled hyperthyroidism.
Once you have been referred for specialist treatment, your specialist will determine the best method of treatment based on your symptoms, and the amount of extra thyroid hormone in your blood. The various methods of treating hyperthyroidism are outlined below.
Thionamides are a common form of treatment for hyperthyroidism. They are a type of medication which stop your thyroid gland producing excess amounts of thyroxine, or triiodothyronine. However, as thionamides work by affecting the production of thyroid hormone, rather than their current levels, you will need to take them for several weeks before.
If your specialist prescribes you a thionamide (either carbimazole or prophylthiouracil), it is likely that you will need to take it for 4-8 weeks before your thyroid gland is under control. However, the amount of medication needed to reduce the production of thyroid hormone varies from person to person, so you may need to take it for a shorter, or longer, period of time than this.
Once the production of thyroid hormone from your thyroid gland is under control, your specialist may gradually reduce the dosage of thionamide. However, if your condition is not under control, you may need to continue taking thionamide long-term. You may also have to take this medication long-term if your overall health prevents you from using other forms of treatment.
In rare cases, you may experience several side effects as a result of taking thionamides, although these are usually mild. These side effects may include:
However, in very rare cases, thionamides can cause a serious condition known as agranulocytosis. If this occurs, it's usually in the first three months of treatment.
You should seek urgent medical attention if you are taking a thionamide and you develop:
Beta-blockers, such as propranolol, or atenolol, can help to relieve some of the symptoms of hyperthyroidism, including tremor (shaking and trembling), rapid heartbeat, and hyperactivity.
Your specialist may prescribe you a beta-blocker in order to relieve your symptoms while you are undergoing a diagnostic assessment, or until your thyroid gland is brought under control by treatment with thionamide. However, beta-blockers are not suitable if you have asthma.
Beta-blockers can sometimes cause side effects which may include:
Radioiodine treatment is a form of radiotherapy that is used to treat most types of hyperthyroidism. It works by targeting tissue in your thyroid gland, and reducing the amount of thyroid hormone it is producing.
Your thyroid gland takes the iodine that you consume from food to make the thyroid hormones, thyroxine and triiodothyronine. Radioiodine treatment contains iodine that is radioactive, 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 discomfort, double vision, or prominent eyes. If you are planning a pregnancy, you should not become pregnant for at least six months after having radioiodine treatment. Men should not father a child for at least four months after having radioiodine treatment.
Surgery to remove either all, or part, of the thyroid gland is a permanent cure for recurrent hyperthyroidism, and is known as a total, or near-total, thyroidectomy.
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:
The goal of the surgery is to remove just enough of your thyroid gland in order to return the production of thyroxine to normal. If too much of the thyroid gland is taken, you may get an under-active thyroid (hypothyroidism). This happens when not enough thyroid hormones are produced. However, if this occurs, thyroid hormone tablets can be taken to keep your thyroid levels normal.