Releasing Hormone

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. When the levels of TSH are too high, they feed back on the brain to shut down the secretion of TRH. 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.

Releasing hormones are sometimes known as liberins. For example, TRH may be known as thyroliberin. Inhibiting hormones may be known as statins. For example, dopamine (which inhibits prolactin release) may be called prolactostatin.