Overactive thyroid (hyperthyroidism)

Published by BUPA's Health Information Team
February 2004
http://hcd2.bupa.co.uk/fact_sheets/html/overactive_thyroid.html

Hyperthyroidism means having an overactive thyroid gland. It results in an excess of the thyroid hormones, making the body's functions "speed up" and causing symptoms such as restlessness, anxiety and weight loss.

Approximately 1 in 50 women and 1 in 1000 men develop hyperthyroidism each year in the UK. Once diagnosed, hyperthyroidism can usually be treated successfully.

The thyroid gland

The thyroid gland is found in the neck. It lies in front of the wind-pipe (trachea), above the level of the collarbones and measures approximately 2 by 4 cm. It secretes hormones, which are chemicals produced by the body to help regulate how cells, and the organs made up of those cells, work. Hormones are sometimes called chemical messengers.

The thyroid hormones are thyroxine (also called T4 because it contains four iodine atoms) and triiodothyronine (also called T3 because it contains three iodine atoms). These are released into the blood stream.

T3 speeds up the body's metabolism by encouraging the cells, in the muscles or the skin for instance, to work faster or to grow. Most of the T3 in the blood is converted from T4.

Over and under-active thyroid

With hyperthyroidism, over-production of T3 and T4 can cause the body's metabolism to speed up resulting in characteristic symptoms. In contrast, hypothyroidism results when the thyroid is underactive and produces too little thyroid hormone. This slows down the body's metabolism. For more information on hypothyroidism please see the separate BUPA factsheet Underactive thyroid (hypothyroidism).

The production of T3 and T4 is regulated by another hormone called TSH (thyroid stimulating hormone or thyrotropin) which is made in the pituitary gland (found in the brain). A normal level of TSH is one of the indicators that the thyroid system is working properly.

Causes of hyperthyroidism

Graves' disease

This is the most common cause of hyperthyroidism in the UK, accounting for 60 to 80% of cases. Graves' disease is an "autoimmune" disease. Antibodies are proteins designed to defend the body from foreign organisms, such as bacteria and viruses. In autoimmune conditions, antibodies attack the body itself. In Graves' disease, antibodies cause the thyroid gland to produce too much thyroid hormone.

Nodular thyroid disease

Nodules - small lumps - within the thyroid account for the majority of the other cases of hyperthyroidism. Abnormal thyroid tissue within these nodules produces too much thyroid hormone. If one nodule forms, this is called toxic solitary adenoma. If more than one forms, this is called toxic multinodular goitre.

Other causes

There are other rare causes of hyperthyroidism. In thyroiditis, there is inflammation of the thyroid gland from either infection or autoimmune attack. Thyroiditis can also be associated with pregnancy. Too much iodine in the diet (or taking drugs containing iodine) is a further cause of hyperthyroidism.

Risk factors

Being female - women are 10 times more likely to develop an overactive thyroid than men.
Graves' disease is more likely in people whose close relatives are affected.
Having other autoimmune diseases, such as type 1 diabetes, vitiligo and Addison's disease, increases the chance of getting hyperthyroidism.

Symptoms

Hyperthyroidism can cause many different symptoms and the overall pattern can vary for different people, depending on their age and the amount of excess hormone being released. In general, the symptoms relate to a "speeding up" of the body's metabolism and they include the following:

feeling nervous, irritable or emotional
tremor (shaking)
sleeping poorly
being intolerant of heat and sweating more than usual
losing weight despite increased appetite (10% of people gain weight)
feeling tired and having weak muscles
increased heart rate or palpitations
having infrequent periods or problems with fertility
having more frequent bowl movements or diarrhoea
shortness of breath, especially when exercising
loss of hair or finer hair than usual
swelling of the thyroid gland in the neck - a goitre

The eyes are affected in approximately 50% of people with Graves' disease. They are pushed forwards and look more prominent. This can lead to changes in eyesight including double vision as well as eye discomfort and watering.

Most people with hyperthyroidism don't get all the symptoms, but they may have a combination of two or more.

Diagnosis of hyperthyroidism

Many of the above symptoms are quite general and could be caused by conditions other than an overactive thyroid. However, anyone who notices these symptoms should consult their GP. A doctor will usually discuss symptoms, carry out a physical examination, and then request some blood tests if he or she suspects hyperthyroidism.

Blood tests

Hyperthyroidism can be confirmed relatively easily by doing some blood tests, although the underlying cause may need a little more investigation. A blood test to determine the amount of thyroid stimulating hormone (TSH) is usually done first. If this is normal, T3 and T4 are also likely to be normal. However, if the TSH is abnormal, T4 and T3 levels may also need to be checked. In some circumstances the presence of certain antibodies may be confirmed with another blood test.

Other tests

The thyroid gland itself may be checked using a test called scintigraphy. This involves injecting a radioactive form of iodine. The thyroid uses this iodine to make T3 and T4, which means certain parts of the gland take up the radioactive material and can be seen with a scanner. The dose of radioactivity is very low and is not dangerous.

Ultrasound scanning can help measure the size of a goitre and whether it is pressing on neighbouring tissues in the neck. CT or MRI scanning may also be needed to assess eye problems.

Treatment of hyperthyroidism

In a minority of cases, hyperthyroidism clears up on its own. But in most cases, it needs treating and tends to recur if treatment is stopped.

Treatment aims to bring thyroid hormones down to normal levels - this state is known as being "euthyroid".

Treatments for hyperthyroid include:

Antithyroid drugs

These drugs suppress the production of thyroid hormones. Examples include carbimazole and propylthiouracil. Too much of these drugs can cause hypothyroidism and regular blood tests are needed to check the correct amount has been taken. Another way of giving these drugs is called the block - replace regimen, where output of the thyroid is completely suppressed and replacement thyroxine (T4) is given in tablet form (thyroxine replacement therapy).

An episode of Graves' disease can get better after 1-2 years of treatment, although relapses are common once treatment is stopped.

Radioiodine (iodine-131)

This is iodine that has been made radioactive, similar to the iodine used for a scintigraphy scan. The body uses iodine to make T3 and T4, so when the radioiodine is given as a tablet, or drink, it is taken up into the thyroid gland. As the radioactivity builds up in the thyroid gland, it destroys some of the thyroid tissue. If just the right amount is given, it may be possible to return the thyroid hormones to the correct level, but this can be difficult.

Sometimes, a single large dose of radioiodine is given, with the intention of stopping all thyroid activity, followed by thyroxine replacement therapy. The dose of radioactivity to the rest of the body is low and is not dangerous. However, radioiodine is not used in pregnant or breastfeeding women.

Surgery

This is used to remove a single nodule (toxic solitary adenoma). It may also be used when there is a large goitre or if drug treatment has failed.

As with any surgery, there are risks. These include bleeding, infection or damage to nearby nerves or other tissues. The surgeon will discuss these with each patient before surgery.

Beta-blockers

Drugs called beta-blockers (eg propranolol or atenolol) are sometimes used to treat the symptoms of hyperthyroidism, for example while waiting for a treatment such as radioiodine to become fully effective.

Treatment for eye problems

Some people need treatment for eye symptoms relating to Graves' disease. These include eye drops, sunglasses or eye protectors for sleeping. Treatment for more severe eye problems might include steroid tablets, radiation treatment or surgery.

Further information

British Thyroid Foundation
0870 7707933
http://www.btf-thyroid.org