It:Thyroxine
Thyroxine (tetraiodothyronine, T4) and triiodothyronine (T3) are hormones produced by the thyroid gland that are essential for regulating metabolism.
A lot more T4 is produced than T3 because it has a longer half-life. It is converted to the more potent T3 (about four times more potent than T4) by a deiodination reaction.
Structure1
Molecular formula – C15H11I4NO4
Molecular Weight – 776.8742
Releasing thyroxine2
Thyrotrophin-releasing hormone is released in the hypothalamus. It is then carried to the pituitary gland to promote the release of thyroid-stimulating hormone. When the thyroid-stimulating hormone reaches the thyroid gland, through travelling in the blood, it can stimulate the release of T4 and T3.
Mechanism for thyroxine production3
Effects of Thyroxine4
Both T4 and T3 increase the basal metabolic rate – so when raised levels are present there may be an increase in body temperature and sensitivity to heat. When reduced levels are present there is a decreased tolerance to cold. Increased levels of T4 and T3 mean that there is increased blood flow to the skin - to reduce the body’s raised temperature.
T3 and T4 increase the metabolism of lipids, carbohydrates and proteins. So raised levels result in a decrease in body mass, and reduced levels result in an increase in body mass.
High levels of T4 and T3 are involved in the resorption of bone, which causes demineralisation and increases the risk of fracture.
T3 and T4 are also essential for mental development.
Converting T4 to T33
T4 is converted into T3 by the deiodination of the outer ring, or rT3 by the deiodination of the inner ring. rT3 (reverse T3) is an inactive form of thyronine whose function is not fully understood.
References
1 http://www.emolecules.com/cgi-bin/search?t=ex&q=thyroxine
2 Chapter 3: How the Thyroid Works, Thyroid for Dummies, Alan Rubin
3 www.siouxvalley.org/classlibrary/page/_staticinclude/allard.ppt