<P> (Coupling of DIT to MIT in the opposite order yields a substance, r-T, which is biologically inactive.) Hydrolysis (cleavage to individual amino acids) of the modified protein by proteases then liberates T3 and T4, as well as the non-coupled tyrosine derivatives MIT and DIT . The hormones T and T are the biologically active agents central to metabolic regulation . </P> <P> Thyroxine is believed to be a prohormone and a reservoir for the most active and main thyroid hormone T. T is converted as required in the tissues by iodothyronine deiodinase . Deficiency of deiodinase can mimic hypothyroidism due to iodine deficiency . T is more active than T, though it is present in less quantity than T . </P> <P> Thyrotropin - releasing hormone (TRH) is released from hypothalamus by 6--8 weeks, and thyroid - stimulating hormone (TSH) secretion from fetal pituitary is evident by 12 weeks of gestation, and fetal production of thyroxine (T) reaches a clinically significant level at 18--20 weeks . Fetal triiodothyronine (T) remains low (less than 15 ng / dL) until 30 weeks of gestation, and increases to 50 ng / dL at term . Fetal self - sufficiency of thyroid hormones protects the fetus against e.g. brain development abnormalities caused by maternal hypothyroidism . </P> <Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section 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>

Mention the four important function of thyroxine hormone