First principle: frequency is the message
The hypothalamus releases GnRH in pulses. The pituitary is a frequency detector, not just an amplitude detector:
- Pulsatile GnRH → sustained FSH/LH secretion (the axis is ON).
- Continuous GnRH → receptor downregulation → FSH/LH fall (the axis is OFF).
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Turning physiology into therapy
This is not trivia — it is a drug class:
- GnRH agonists (continuous exposure) cause an initial flare, then medical castration — used to shrink fibroids pre‑op, suppress endometriosis, and downregulate the axis in IVF.
- Pulsatile GnRH (a pump) can induce ovulation in hypothalamic failure.
Discriminators: three feedback modes
| Setting | Estrogen feedback | Result |
|---|---|---|
| Most of the cycle | negative | restrains FSH/LH |
| Late follicular (sustained high E₂) | positive | triggers the LH surge |
| Menopause / POI | no estrogen | FSH/LH rise unopposed |
Pitfalls
- Expecting a GnRH agonist to suppress immediately — the initial flare can transiently worsen a hormone‑sensitive condition; antagonists or add‑back may be used.
Pearls
- 🎯 Pulsatile stimulates; continuous suppresses. One sentence explains an entire drug class.
- 🩺 GnRH‑agonist "add‑back" (low‑dose estrogen/progestin) protects bone during prolonged suppression.
Next → The pituitary sends two gonadotropins to the ovary — how do they cooperate? (Concept 6).