The thyroid gland contains a large number of follicles that release and store several thyroid hormones. All are derivatives of the amino acid, tyrosine, to which three or four iodine atoms have been attached.
Thyroid hormones affect almost every cell in the body by stimulating:
- cellular production of heat
- protein synthesis
- lipid synthesis, mobilization, and degradation
- carbohydrate metabolism
- synthesis of coenzymes from vitamins
- responses of tissues to epinephrine and norepinephrine
- the development of all body systems especially skeletal, muscular, and nervous systems
Despite its name, TSH (Thyroid stimulating hormone) is secreted by the anterior pituitary gland.
- Thyroxine (TX) is also known as T4 or tetraiodothyronine because it has four atoms of iodine attached. TX accounts for almost 90% of all thyroid secretions. The serum level is normally 45 to 50 times the level of T3 (triiodothyronine). Despite T3 containing only three iodine atoms, it is several times more active than T4. Thyroxine:
- regulates the metabolism;
- speeds up the activity of all body organs except the brain, testes, spleen, and anterior pituitary.
- Calcitonin keeps blood calcium levels from increasing abnormally.