positive feedback

Positive feedback is the opposite of a thermostat: instead of damping a change, it amplifies it. The output of a process feeds back to increase its own stimulus, so the response grows stronger and stronger — like a microphone placed too close to its speaker producing a louder and louder squeal. Because it is self-reinforcing, the body uses it only for short, decisive events that need to run to completion.

The textbook endocrine example is the LH surge at mid-cycle. Rising estradiol from the maturing ovarian follicle, instead of suppressing the hypothalamus and pituitary as it usually does, switches to stimulating them; this drives a sharp burst of LH that triggers ovulation. Another example is oxytocin during labor: stretching of the cervix stimulates oxytocin release, oxytocin strengthens uterine contractions, contractions stretch the cervix further, and the cycle escalates until delivery.

Positive feedback is inherently unstable, so it must have a built-in endpoint that breaks the loop — ovulation removes the follicle that was driving the surge, and birth removes the cervical stretch driving oxytocin. Without such a stopping condition, runaway positive feedback would be harmful, which is why negative feedback remains the everyday default.