Porcine thyrotropin releasing hormone is (pyro)glu-his-pro(NH2).

Porcine thyrotropin releasing hormone, isolated from the hypothalamus, and synthetic L-(pyro)Glu-L-His-L-Pro(NH2) are biologically identical. (pyro)Glu-His-Pro(NH2) is active in vivo at 1 ng in mice. It stimulates TSH release from rat anterior pituitary glands in vitro. The in vivo and the in vitro responses are inhibited by triiodothyronine. (pyro) Glu-His-Pro (NH2) raises plasma TSH levels in rats and mice as well as hypophysectomized rats with pituitary transplants under the renal capsule. When incubated in human serum it is inactivated, (pyro) Glu-His-Pro (NH2) is active orally as well as ip, iv and sc in mice. Minor modifications in the (pyro) Glu-His-Pro (NH2) chemical structure decrease its biological activity. (Endocrinology 86: 1143, 1970)