releasing hormone

One of a group of hormones produced in different parts of the hypothalamus, and stored and released by the neurones of the median eminence of the hypothalamus in response to stimuli from the brain. It is transported to the front lobe of the pituitary gland, where it stimulates or inhibits the release of a specific pituitary hormone (eg growth-hormone-releasing hormone). Previously known as a releasing factor, this term is now confined to hypothalamic hormones whose chemical structures are unknown (eg prolactin-releasing factor).

A releasing hormone or releasing factor is a hormone whose main purpose is to control the release of another hormone. The main releasing hormones secreted by the hypothalamus are:

Thyrotropin-releasing hormone (TRH), Corticotropin-releasing hormone (CRH), Gonadotropin-releasing hormone (GnRH), Growth hormone-releasing hormone (GHRH),

Two other factors are also classed as releasing hormones, although they in fact inhibit pituitary hormone release

Somatostatin, Dopamine.

For example, TRH is released from the hypothalamus in response to low levels of secretion of thyroid stimulating hormone(TSH) from the pituitary gland. The TSH in turn is under feedback control by the thyroid hormones T4 and T3. Synthetic TRH is also used by physicians as a test of TSH reserve in the pituitary gland as it should stimulate the release of TSH and prolactin from this gland.