A releasing hormone or releasing factor is a hormone whose sole purpose is to control the release of another hormone. These are also called hypophysiotropic or hypothalamic hormones. The use of the term factor was employed for some time pending the establishment of the molecular structure of the hormones. When this had been fully established the hormones were referred to as releasing hormones. 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.
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. 
Releasing hormones increase (or, in case of inhibitory factors, decrease) the intracellular concentration of calcium (Ca2+), resulting in vesicle fusion of the respective primary hormone.
For GnRH, TRH and GHRH the increase in Ca2+ is achieved by the releasing hormone coupling and activating G protein coupled receptors coupled to the Gq alpha subunit, activating the IP3/DAG pathway to increase Ca2+. For GHRH, however, this is a minor pathway, the main one being the cAMP dependent pathway. 
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, Roger. Hypothalamic hormones aka hypothalamic releasing factors. J Endocrinol Jan 1 2005 184 11-28 doi.10.1677/joe.1.05883
- Page 237 in: Costanzo, Linda S. (2007). Physiology. Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN 0-7817-7311-3.
- Walter F., PhD. Boron (2003). Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders. p. 1300. ISBN 1-4160-2328-3.
- Guillemin R. Hypothalamic hormones a.k.a. hypothalamic releasing factors. J Endocrinol 2005;184:11-28. Fulltext. doi:10.1677/joe.1.05883 PMID 15642779.