Negative Feedback Loop

In a negative feedback loop, a rise in the output signal acts back on the steps that generated it to reduce further production. Three properties determine behaviour: the set point the loop defends, the gain, which fixes how strongly a deviation is corrected, and the delay between output and correction. A long delay with high gain produces oscillation rather than steady control, which is one reason several endocrine axes are pulsatile rather than tonic by design.

The endocrine examples are the clearest. Growth hormone drives hepatic production of IGF-1, and IGF-1 in turn inhibits growth hormone release at the pituitary while stimulating hypothalamic somatostatin. Testosterone and estradiol suppress luteinising hormone and follicle-stimulating hormone. Thyroid hormones suppress thyroid-stimulating hormone so tightly that TSH is the most sensitive indicator of thyroid status. Glucocorticoids suppress corticotropin-releasing hormone and ACTH, which is why prolonged exogenous steroid treatment requires tapering rather than abrupt cessation.

Feedback is what distinguishes stimulating a system from replacing its hormone. Anything acting inside the loop, such as a secretagogue, has its effect bounded by the loop's own correction, so response is self-limiting and tends to blunt with continued stimulation. A hormone given from outside bypasses the sensor entirely, raises output beyond what the loop would permit, and suppresses the endogenous arm.

The claim that misuses this is the one asserting that stimulating your own production cannot cause suppression because it is natural. Feedback does not care about origin; it responds to the level. The mirror error is assuming replacement doses of an exogenous hormone leave the axis untouched, when suppression is the expected consequence of raising the regulated variable at all.