<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (April 2016) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (April 2016) (Learn how and when to remove this template message) </Td> </Tr> <P> The thyroid gland makes and releases two hormones: triiodothyronine (T) (o - (4 - Hydroxy - 3, 5 - iodophenyl) 3, 5 - diiodophenyl tyrosine) and thyroxine (T) (o - (4 - Hydroxy - 3, 5 - diiodophenyl) 3, 5diiodophenyl tyrosine). They are tyrosine - based hormones that are primarily responsible for regulation of metabolism . T and T are partially composed of iodine (see molecular model). A deficiency of iodine leads to decreased production of T and T, enlarges the thyroid tissue and will cause the disease known as simple goitre . The major form of thyroid hormone in the blood is thyroxine (T), which has a longer half - life than T. In humans, the ratio of T to T released into the blood is sometimes claimed to be quite high, but thyroid removal patient data suggests it to vary between 4: 1 to 2: 1, the average being 100: 36 (roughly 2.8: 1). T is converted to the active T (three to four times more potent than T) within cells by deiodinases (5' - iodinase). These are further processed by decarboxylation and deiodination to produce iodothyronamine (T a) and thyronamine (T a). All three isoforms of the deiodinases are selenium - containing enzymes, thus dietary selenium is essential for T production . Edward Calvin Kendall was responsible for the isolation of thyroxine in 1915 . </P> <P> The thyroid hormones act on nearly every cell in the body . They act to increase the basal metabolic rate, affect protein synthesis, help regulate long bone growth (synergy with growth hormone) and neural maturation, and increase the body's sensitivity to catecholamines (such as adrenaline) by permissiveness . The thyroid hormones are essential to proper development and differentiation of all cells of the human body . These hormones also regulate protein, fat, and carbohydrate metabolism, affecting how human cells use energetic compounds . They also stimulate vitamin metabolism . Numerous physiological and pathological stimuli influence thyroid hormone synthesis . </P>

What is the purpose of t3 and t4