Negative Feedback Loop
A self-regulating biological mechanism where the output of a hormonal system inhibits its own production, maintaining homeostasis. Many peptide drugs interact with negative feedback loops — for example, exogenous growth hormone suppresses endogenous GH production through negative feedback via IGF-1.
Technical Context
In the GH axis: GH stimulates hepatic IGF-1 production; IGF-1 feeds back to suppress hypothalamic GHRH and pituitary GH release. Exogenous GH (somatropin) suppresses endogenous GH through this loop. In the HPG axis: testosterone/oestrogen feed back to suppress hypothalamic GnRH and pituitary LH/FSH. GnRH agonist-induced testosterone suppression removes this feedback, which is why LH initially rises (flare) before receptor downregulation takes effect. In the HPA axis: cortisol feeds back to suppress hypothalamic CRH and pituitary ACTH. Understanding feedback loop dynamics is essential for predicting both therapeutic effects and potential complications of peptide hormone therapies.