Underactive thyroid (hypothyroidism) - occurs when the thyroid gland does not produce enough thyroid hormones. This tends to slow down the body’s functions. Symptoms include tiredness, constipation and sensitivity to the cold.

The thyroid gland is found in the neck. It lies in front of the wind-pipe (trachea), above the level of the collar-bones 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 groups of 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 underactive thyroid
In hypothyroidism, the thyroid is under active, with too little of the thyroid hormones being released. In contrast, hyperthyroidism occurs when there is over-production of T3 and T4, causing the body’s metabolism to "speed up". This results in symptoms such as anxiety and feeling hot, even in cool weather. For more information, please see the separate BUPA factsheet Overactive thyroid (hyperthyroidism).

Hormone production
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 hypothyroidism
Iodine insufficiency
The chemical element iodine is a major component of thyroid hormones. Worldwide, a deficiency of iodine in the diet is the leading cause of hypothyroidism. This is much less common in the UK because iodine is contained in salt, which has iodine added to it during manufacture.

Autoimmune hypothyroidism
In the UK, and other countries where the amount of iodine in the diet is adequate, the main cause of hypothyroidism is "autoimmune hypothyroidism".

Antibodies are proteins designed to defend the body from foreign organisms, such as bacteria and viruses. In autoimmune diseases, antibodies attack the body itself. In autoimmune hypothyroidism, antibodies destroy thyroid gland cells preventing the gland from being able to release normal amounts of thyroid hormones.

A condition known as Hashimoto’s thyroiditis is associated with autoimmune hypothyroidism. It results in a goitre, a swelling of the thyroid gland, that is visible as a lump on the neck. With no goitre the condition may be called atrophic thyroiditis or primary myxoedema.

Other causes:
Congenital hypothyroidism (a condition babies are born with) happens when the thyroid gland fails to develop properly or does not produce adequate thyroid hormones. All babies in the UK are screened for this when the baby is 6-8 days old with a blood sample taken from a prick on the heel. With treatment, babies with congenital hypothyroidism develop normally.

Inflammation of the thyroid gland (thyroiditis) due to infection can lead to hypothyroidism. Also, disorders of the hypothalamus and pituitary gland, both of which are involved in the overall regulation and production of thyroid hormones, can lead to hypothyroidism.

Risk factors for hypothyroidism
Hypothyroidism is more common in older people.
Women are more likely to be affected than men.
Autoimmune hypothyroidism is more likely in those who have other conditions resulting from an autoimmune disorder such as type 1 diabetes mellitus, vitiligo and Addison’s disease.
Some medicines can affect the normal functioning of the thyroid gland. These include lithium carbonate (for bipolar disorder) and amiodarone (for heart rhythm abnormalities).
Symptoms of hypothyroidism
The symptoms of hypothyroidism can initially be very mild and develop slowly. It is possible to have some of these symptoms before the amount of thyroid hormone drops below normal.

This type of mild hypothyroidism is called subclinical hypothyroidism. People affected need to be monitored by their doctor, who will watch out for further symptoms.

Occasionally, a thyroid problem, such as thyroiditis, can resolve on its own without the need for treatment. However in general, if hypothyroidism is not treated, the symptoms slowly get worse and it becomes more and more difficult to function normally.

Diagnosis of hypothyroidism
Many of the above symptoms can be caused by conditions other than underactive thyroid. However, anyone who experiences these symptoms should consult their GP. A doctor will usually discuss symptoms, perform a physical examination and then request some blood tests if he or she suspects hypothyroidism.

The first step in diagnosing hypothyroidism is to measure TSH (thyroid stimulating hormone).

When the thyroid gland is not producing enough thyroid hormone, the TSH level is raised. When this abnormality is found, T4 (thyroxine) is also measured, and in autoimmune hypothyroidism or hypothyroidism due to treatment of hyperthyroidism, T4 is low.

When levels of TSH and thyroid hormones are difficult to interpret, other causes of hypothyroidism may be suspected.

The diagnosis of autoimmune hypothyroidism is usually confirmed by the presence of particular antibodies in the blood.

Other blood tests or further investigations may also be needed. These could include an ECG (heart tracing) or a magnetic resonance imaging (MRI) scan to examine the nature and extent of a goitre.

Treatment
Thyroxine (T4) can be given in tablet form. The body is able to convert this to T3 just as it would if the thyroid gland were producing the thyroxine normally. It can take some time to get the dose right. It is usual to start with a low dose, building up gradually every six weeks and adjusting the dose according to TSH levels.

People usually feel much better once they are taking thyroxine. Side-effects are unusual because a missing hormone is simply being replaced. However, if too much replacement thyroxine is given, symptoms of an overactive thyroid may be experienced.

People with subclinical hypothyroidism may not have any treatment, though doctors vary in their approach. Some prefer to offer treatment. Others recommend frequent monitoring to see whether overt hypothyroidism (with symptoms) does develop.

Once the correct dose of thyroxine replacement has been established, it is usual to have annual checks of TSH.

Pregnancy and hypothyroidism
The functioning of the thyroid gland can change during pregnancy. In the first half of pregnancy it is normal for the total amount of thyroid hormone to be slightly increased.

Women with hypothyroidism need more frequent checks during pregnancy, as their thyroxine requirements tend to increase.

Five percent of women have a mild problem with the thyroid three to six months after delivery. This has an autoimmune cause and can cause hyperthyroidism or hypothyroidism. Although it rarely needs treatment it does mean that thyroid problems are more likely in later life.

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

Often the symptoms arise gradually. Your doctor can confirm the diagnosis by measuring levels of thyroid hormones in the blood.