TRH free acid is derived from Thyrotropin Releasing Hormone (TRH), and the latter could cause the release of Thyroid Stimulating Hormone (TSH) from the pituitary and was isolated in pure form and the structure determined. They are different functionally, such as stimulation of GTPase depended on occupancy of the TRH receptor; half-maximal increases in GTPase activity required 46 nM TRH and 25 nM [N3-methyl-His]TRH, but the TRH free acid was inactive.