Thyroid hormones are derivatives of tyrosine (an amino acid) bonded covalently to iodine. The principal thyroid hormones are thyroxine (also called T4) and triiodotyronine (also called T3). These hormones are essential formed by the linking together of two tyrosine molecules with the addition of three or four iodine atoms covalently bonded to the aromatic rings of these structures. The number of iodine atoms as well as their positions are extremely important as they ultimately determine the nature of the resulting molecule. That is to say that there are a few such iodinated molecules formed that have no biological activitty whatsoever, such as reverse T3.
The thyroid gland secretes mostly T4, however T3 is a much more potent and biologically active hormone. Some T3 is also secreted but most of the T3 found in the blood is formed by the deiodination of T4, a process that mainly occurs in the liver and the kidneys. This process also often yields reverse T3.
Thyroid hormones are not very soluble in water and most T3 and T4 is bound to carrier proteins, most noteably thyroid-binding globulin, a glycoprotein. These carrier proteins allow there to be a stable amount of free, unbound thyroid hormones circulating in the blood to be uptaken by target cells by releasing the active hormones periodically.
The synthesis and secretion of thyroid hormones is regulated by thyroid-stimulating hormone (TSH). TSH is released from the anterior pituitary and travels to the thyroid gland where it binds to target cells.
It is very probable that all cells in the body are targets for thyroid hormones. So, it seems logical that these hormones should have many physiological effects, and they do. Probably among the most obvious effects is an increase in metabolic activity resulting in an increase in basal metabolic rate. A result of this is an increase in body temperature as many users of T3 have noted.
These hormones also impact growth. This is seen quite clearly in the growth retardation of children who are deficient in thyroid hormones. They can also increase protein synthesis to a degree.
Other effects of these hormones include the development of the brain during early stages of life, increased heart rate, increased vasolidation, effects on mental well-being, and infertility in peeps with hypothyroidism.
Alright, I'm tired and am going to leave it at that. Bump if you appreciate this type of post.
