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
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.
Roger Guillemin and Andrew W. Schally were awarded the Nobel Prize in Physiology and Medicine in 1977 for their contributions to understanding "the peptide hormone production of the brain"; these scientists independently first isolated TRH and GnRH and then identified their structures.
- Guillemin R. Hypothalamic hormones a.k.a. hypothalamic releasing factors. J Endocrinol 2005;184:11-28. Fulltext. PMID 15642779.