Thyroid releasing hormone, TRH, is a tripeptide that is released by the hypothalamus which stimulates the production of thyroid stimulating hormone, TSH, a polypeptide, from the anterior pituitary. TSH in turn stimulates the release of the thyroxines (T4 and the more potent T3) from the thyroid gland.
The thyroid gland produces predominantly T4 with a small amount of T3. About 85% of circulating T3 is the result of monodeiodination of T4 in the tissues, especially the liver, muscle and kidney. T3 is about five times more active than T4. The majority of T3 and T4 is carried in the plasma bound to thyroxine binding globulin, TBG, thyroxine binding pre-albumin, TBPA, and albumin. However, unbound T3 and T4 are the active forms and gain entry into the cell by an ATP dependent process.
Production of T3 and T4 is controlled primarily, by the circulating concentrations of free thyroid hormone which exert a negative feedback on TSH release. However, dopamine, somatostatin and glucocorticoids may reduce plasma TSH and so affect TSH release.
Both T3 and T4:
- increase cell metabolism
- facilitate normal growth
- facilitate normal mental development
- increase the local effects of catecholamines
The half-life of T3 is approximately 24 hours whereas the half-life of T4 is about 7 days.