Thyroid Gland

The thyroid gland is located at the base of the throat. The gland curves across the anterior surface of the trachea just below the thyroid cartilage. The two lobes are joined by a slender connection called the isthmus.

Size of the gland depends on heredity, environment, and nutritional factors. Although quite variable, the average weight is about 34 grams or 1.2 ounces. It is a deep red colour because of the large number of blood vessels servicing the glandular cells.

Thyroid Hormones
The thyroid gland contains a large number of follicles that stores the thyroid hormones. Thyroid follicles release several 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 (see 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.

This page was updated in November 2005.