Secretagogue.

A substance that triggers secretion of another substance.

A secretagogue is anything that causes a gland or cell to release a substance. The term appears constantly in this field as growth-hormone secretagogue, covering compounds that prompt the pituitary to release growth hormone, and it includes both GHRH analogs acting on the GHRH receptor and GHRPs acting on the ghrelin receptor, which is formally named the growth-hormone-secretagogue receptor. The concept is worth having because it names a general strategy: rather than supplying a hormone from outside, you prompt the body to release its own. That approach preserves the natural pulsatile release pattern and leaves the existing feedback loops intact, so the system retains some ability to limit itself, which is the main argument made for it over direct hormone administration. The limits of that argument deserve stating plainly. A secretagogue can only work if the gland is capable of secreting in the first place, so it cannot substitute for genuine deficiency. Feedback that limits a response is not the same as an absence of risk. And receptors desensitize under sustained stimulation, so a secretagogue's effect can diminish with continuous use in a way that a direct hormone supply would not.