Negative Feedback (Endocrine)
Endocrine negative feedback is the loop in which a target gland's hormone suppresses the hypothalamic and pituitary signals that drive it, holding circulating concentrations near a set point.
Negative feedback is the control architecture of every classical endocrine axis. A hypothalamic releasing hormone drives a pituitary tropic hormone, which drives a peripheral gland, whose product then inhibits both upstream stations - the long loop. Shorter loops exist: a pituitary hormone can inhibit its own hypothalamic driver, and some cells inhibit their own secretion outright. The result is a servo system whose regulated variable is the peripheral hormone rather than the tropic one.
The gain of these loops is steep and non-linear. Free T4 and TSH are related roughly log-linearly, so a free T4 shift too small to leave the reference range can move TSH several-fold, which is what makes TSH the most sensitive single index of thyroid status. The gonadal and adrenal axes behave the same way: exogenous testosterone drives LH and FSH to undetectable within weeks, and supraphysiological glucocorticoid suppresses CRH and ACTH until the adrenal cortex atrophies.
Feedback is also why the upstream hormone is the diagnostic one. Pairing a peripheral hormone with its tropic partner localises the lesion: low cortisol with high ACTH is adrenal, low with low is pituitary; low testosterone with high LH is testicular, low with low is central. Reading either number in isolation throws away the information the loop encodes.
The error that matters in practice is forgetting that feedback has a time constant. TSH takes about six weeks to re-equilibrate after a levothyroxine change, so a level drawn at two weeks invites a dose adjustment chasing a number still in transit. The same lag runs the other way once exogenous hormone stops: suppression is fast, recovery is slow, and a gonadotropin measured shortly after a suppressive course ends says nothing about whether the axis will restart.