Underactive thyroid (hypothyroidism)

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.

Approximately 1 in 50 women and 1 in 1000 men will develop symptoms of hypothyroidism at some stage in their lives. Once diagnosed, treatment is usually straightforward.