Thyrotropin-Releasing Hormone: A Powerful Tripeptide With Diverse Effects in Horses

Thyrotropin-releasing hormone (TRH) was the first trophic “factor” from the hypothalamus to be isolated and identified chemically. It was known to have a stimulatory effect on thyroid-stimulating hormone (TSH) production and secretion by cells within the adenohypothesis, and its structure was revealed to be a modified tripeptide: (pyro)Glu-His-Pro-NH2. Intravenous or intramuscular injection of TRH to horses results in an immediate rise in plasma TSH concentrations, as would be expected. However, it is now known that TRH has consistent effects on four of the other five cell types in the equine adenohypophysis as well. Administration of TRH to horses stimulates not only plasma TSH concentrations, but also plasma prolactin, adrenocorticotropic hormone, and melanocyte-stimulating hormone concentrations. In contrast, administration of TRH simultaneously or within 1 hour before administration of secretagogues for growth hormone (GH) greatly reduces the GH response to the secretagogues. To date, TRH has not been reported to have positive or inhibitory effects on the release of luteinizing hormone or follicle-stimulating hormone from gonadotropes. Whether the noted effects of TRH on cells other than the thyrotropes are physiologic or pharmacologic is not clear. Regardless, a significant clinical utility has developed for the use of TRH in diagnosing thyroid gland disease as well as pituitary pars intermedia dysfunction in horses.