The simple idea (no memorising)
Three floors, top to bottom:
- Hypothalamus (top of the brain's chain) releases GnRH.
- Pituitary (just below) hears GnRH and releases two hormones: FSH and LH.
- Ovary hears FSH and LH and gets to work.
Doctors call this the HPO axis — Hypothalamus, Pituitary, Ovary.
The surprising rule: it must blink, not stare
GnRH works only when it is released in pulses — little blinks, roughly every hour. This is one of the most beautiful facts in the whole subject.
- Pulsing GnRH → keeps the system switched ON.
- Constant, non‑stop GnRH → paradoxically switches the system OFF.
Think of it like knocking on a door. A few clear knocks get a response. Leaning on the doorbell so it rings forever just makes everyone stop listening.
Why this strange fact is useful
Doctors use this on purpose. A drug that floods the body with continuous GnRH‑like signal will, after a short burst, shut the ovary down — handy for calming conditions such as fibroids or endometriosis, which you'll meet later. So this is not a piece of trivia; it's a tool.
Why it matters
Every hormone problem downstream — no periods, too many male hormones, infertility — starts as a question about this axis. Is the brain sending the signal? Is the pituitary answering? Is the ovary listening? You will return to these three floors constantly.
Quick check ✅
- What do the letters HPO stand for?
- What happens if GnRH is given continuously instead of in pulses?
Remember this 🌟
Where we're going next → The pituitary sends two hormones down. What do FSH and LH actually do at the ovary? Concept 6.