Triiodothyronine is a hormone secreted by the thyroid gland, taking part in the body's control of metabolism. It is the most powerful thyroid hormone, and affects almost every process in the body, including body temperature, growth, and heart rate. It is also known as T3.
Triiodothyronine helps regulate growth and development, helps control metabolism and body temperature, and, by a negative-feedback system, acts to inhibit the secretion of thyrotropin by the pituitary gland. Triiodothyronine is produced mainly from the deiodination of thyroxine in the peripheral tissues but is also synthesized by and stored in the thyroid gland as an amino acid residue of the protein thyroglobulin.
Thyroid-stimulating hormone (TSH) activates the production of thyroxine (T4) and triiodothyronine (T3). This process is under regulation. In the thyroid, T4 is converted to T3. TSH is inhibited mainly by T3. The thyroid gland releases greater amounts of T4 than T3, so plasma concentrations of T4 are 40-fold higher than those of T3. Most of the circulating T3 is formed peripherally by deiodination of T4 (85%), a process that involves the removal of iodine from carbon 5 on the outer ring of T4. Thus, T4 acts as prohormone for T3.
Triiodothyronine is similar to thyroxine but with one fewer iodine atoms per molecule. In addition, T3 exhibits greater activity and is produced in smaller quantity.