Structure:
Thyroxine, sometimes called thyroglobulin, and referred to as T4, because
of the four iodine atoms on the molecule. It consists of two, joined tyrosine
amino acids, and is secreted by the colloid of thyroid
glands. Iodine from the diet
is actively trapped by this gland to make T4. Upon release from the gland,
most of the T4 and some T3 are bound to thyroxine-binding globulin in the
blood for selective distribution to target tissues; this protein is formed
in the liver. In target tissues, thyroxine
is converted to triiodothyronine, or T3, a more potent hormone than thyroxine,
by removing one of the iodine atoms.
Function:
Thyroxine is the body's main metabolic hormone, regulating the rate of
oxygen use by cells, and the generation of body heat. In the liver,
T4 promotes glucose metabolism to pyruvate, gluconeogenesis from fats,
and increases release of cholesterol. In the digestive
system, T4 increases secretion of digestive juices, and promotes smooth
muscle contractions for motility. In skeletal
and in cardiac muscles, T4 promotes normal
development, tone and function. In bones,
T4 promotes growth and coordinates maturation with age. In the nervous
system, T4 promotes normal development of the system during youth,
and maintains it's functions during adulthood. In the reproductive
system, T4 promotes the ability for conception, and secretion of milk
by the breasts. In the skin,
T4 promotes hydration, secretions of exocrine glands, and the growth of
hair.
