The HPG Axis and the LH Surge

The chain of command

Behind the cycle sits a three-level control system: the hypothalamic-pituitary-gonadal axis. The hypothalamus releases GnRH in little pulses. GnRH tells the anterior pituitary to release two gonadotropins, LH and FSH. LH and FSH travel to the ovary and drive the follicle (exactly the two-cell system from guide 2). The ovary's hormones — estradiol, progesterone, inhibin — then report back up to the brain. It is a loop, and the direction of that feedback is the heart of this guide.

Mostly negative feedback

For most of the cycle, the loop runs on negative feedback, like a thermostat. When estradiol is at low-to-moderate levels, it tells the brain to ease off — keeping LH and FSH gently restrained so things don't overshoot. Inhibin from the granulosa cells adds a selective brake specifically on FSH. This restraint is useful: it helps a single dominant follicle win. As that one follicle pumps out estrogen and inhibin, FSH falls, and the smaller follicles — which needed more FSH to survive — starve and drop out.

The flip: estrogen turns positive

Here is the twist that makes ovulation possible. If estradiol stays *very high for a sustained stretch* — which only happens when a follicle is fully ripe — the brain's interpretation flips. The same estrogen that was a brake becomes an accelerator: positive feedback. The pituitary now responds to GnRH with a massive, sudden release of LH — the LH surge. This is the one famous place in the female body where positive feedback rules, and it lasts only about a day.

The surge is the trigger. About 24 to 36 hours after the LH peak, the mature follicle ruptures and releases its egg — that is ovulation. (Home ovulation kits detect this LH spike in urine, which is why a positive test predicts ovulation a day or so in advance.) Once the egg is out, the leftover follicle becomes the corpus luteum from guide 3, progesterone rises, and progesterone slams the loop firmly back into negative feedback — locking the door so a second surge can't happen this cycle.

FEEDBACK FLIP ACROSS THE CYCLE

LOW / MODERATE estradiol  --(-)-->  brain holds LH/FSH down
  (negative feedback = the usual setting)

SUSTAINED HIGH estradiol  --(+)-->  BRAIN UNLEASHES LH
  (positive feedback = the rare exception)
            |
            v
        LH SURGE  --(~24-36 h)-->  OVULATION
            |
            v
  progesterone rises  --(-)-->  back to negative feedback
  (door locked for the rest of the cycle)