feedback loop

A feedback loop is the complete circular path of information by which a hormone's output comes back to influence its own production. Think of it as a conversation that loops: a gland sends out a hormone, the hormone (or its effect) is sensed, and that sensing changes how much the gland sends out next. The loop closes the circle so the system can regulate itself rather than running blind.

In endocrinology a feedback loop usually spans several glands. The classic three-tier loop runs hypothalamus to pituitary to a peripheral gland, and then the peripheral hormone loops back to the upper two levels. The loop can be negative (most common, stabilizing) or positive (rare, amplifying), and it can be long (final hormone back to the brain), short (pituitary hormone back to hypothalamus), or ultra-short (a hormone acting on its own gland).

Understanding the loop, not just the individual hormones, is what lets clinicians interpret hormone panels. Measuring a gland's hormone together with its controlling hormone localizes the problem: a low peripheral hormone with a high stimulating hormone points to the gland itself, whereas both being low points to a problem higher up in the loop.