QS Study

Thyroid hormones are hormones produced by the thyroid gland that have roles in metabolism. Thyroid hormones are two hormones produced and released by the thyroid gland, namely triiodothyronine (T3) and thyroxine (T4). They act to increase the basal metabolic rate, affect protein synthesis, help regulate long bone growth and neural maturation, and increase the body’s sensitivity to catecholamines by permissiveness.

The release of the thyroid hormone

(a) Initiation of release by the action of TSH: TSH release from the anterior pituitary gland which initiates the release of thyroid hormone.

(b) Ingestion of colloid by thyroid cell:

By the help of action of TSH, the thyroid cell ingests colloid by endocytosis. The apical surface of thyroid cells sends out pseudopod extensions. These extensions close around small portions of the colloid to form pinocytic vesicles.

(c) Formation of phagolysosome by lysosome:

At the same time, the lysosome fuses with colloid droplet to form “Phagolysosome” in which physiologically active proteinases are released. The apical surface of thyroid cells sends out pseudopod extensions. These extensions close around small portions of the colloid to form pinocytic vesicles.

(d) Liberation of T3, T4, DIT, MIT into cytoplasm:

The peptide bonds between the iodinated residues and the thyroglobulin are broken by proteinase in the lysosomes and T4, T3, DlT, and MIT are liberated into the cytoplasm.

(e) The release of thyroid hormone in the blood: T3 and T4 diffuse into the surrounding capillaries through the base of thyroid cell.

Fig: Release of Thyroid Hormone

(f) Reutilization of iodine for hormone synthesis: The iodinated tyrosine (DIT and MIT) are de iodinated by microtonal enzyme iodotyrosine deiodinase. This enzyme liberated iodine from MIT and DIT. This normally provides about twice as much iodine for hormone synthesis as the iodide pump does. MIT and DIT are not secreted.

The rate of secretion: The human secretes about:

  • 80 μg (103 nmol) of T4
  • 4 μg (7 nmol) of T3
  • 2 µg (3.5 nmol) of RT3 per day.