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Triiodothyronine, also known as T3, is the main active form of thyroid hormone. It is produced by the thyroid gland together with the hormone thyroxine, or T4. Only a relatively small proportion of the total T3 circulating in the blood comes directly from the thyroid, with more than 80 percent being formed in the body tissues by removing iodine from thyroxine. Thyroid hormones are thought to affect practically all the cells in the body and the effects of T3 include the stimulation of metabolism, growth and development. This means that the body's energy levels, temperature and the proper function of all its organs and tissues depend on normal triiodothyronine function.
Thyroid hormones are made from molecules of tyrosine, a type of amino acid, and iodine. Thyroxine, or T4, is made of two tyrosines and four iodine atoms while triiodothyronine, or T3, also consists of two tyrosines but with only three iodine atoms joined on. Removal of an iodine atom converts T4 into the active hormone T3, and this is the process that takes place in body tissues. In the blood, most of the thyroid hormones are bound to a special carrier protein called thyroxine-binding globulin but there are also small amounts of unbound, or free, T3 and T4.
The pituitary gland in the brain releases a hormone called thyroid stimulating hormone, or TSH, which acts on the thyroid gland causing it to make triiodothyronine and thyroxine and release them into circulation. A fall in the amount of free hormones causes the pituitary to react by increasing TSH, stimulating the thyroid gland to release more hormones. If the amount of free hormones rises, TSH will fall and the thyroid will produce fewer hormones. In this way the amount of free thyroid hormones in the circulation is constantly regulated.
Disease of the thyroid gland can cause the production of thyroid hormones to increase or decrease abnormally, leading to the conditions known as hyperthyroidism and hypothyroidism, respectively. In hypothyroidism, there is less triiodothyronine acting on body cells and the metabolic rate decreases giving symptoms of tiredness and feeling cold. Hyperthyroidism sends the metabolism into overdrive, causing symptoms such as restlessness, weight loss and diarrhea.
Normally, blood tests including TSH and free T4 are carried out, but a free T3 measurement may also be required if TSH is abnormally low but T4 is normal. This is needed to detect the condition known as T3-thyrotoxicosis where the thyroid produces excessive amounts of T3, while T4 production remains normal. Free T3 levels can be raised, together with free T4 levels in the most common form of hyperthyroidism, known as Grave's disease. Treatment of hyperthyroidism may involve medication, radiotherapy or surgical removal of the thyroid gland. Hypothyroidism is usually treated by taking thyroid hormone, most often in the form of T4, but sometimes T3 is used, or a combination of the two.