Step by Step
1
GnRH — pulsatile release is essential
The hypothalamus releases gonadotropin-releasing hormone (GnRH) in pulses, and this pulsatile pattern is essential for normal function. Continuous (non-pulsatile) GnRH paradoxically suppresses the entire axis by downregulating receptors — this is actually used clinically: GnRH agonists given continuously treat endometriosis, prostate cancer, and precocious puberty by deliberately shutting the system down.
2
FSH and LH from the anterior pituitary
GnRH stimulates the anterior pituitary to release FSH and LH. FSH drives follicle development in females and spermatogenesis/Sertoli cell support in males. LH triggers ovulation and supports the corpus luteum in females, and stimulates testosterone production via Leydig cells in males.
3
Sex hormone feedback — mostly negative, with one key exception
Sex hormones (estrogen, progesterone, testosterone) typically feed back NEGATIVELY, suppressing GnRH, FSH, and LH. The major exception: at mid-cycle in females, high estrogen levels create positive feedback instead, triggering the LH surge that causes ovulation.
4
Inhibin — a selective inhibitor
Inhibin, produced by Sertoli cells (in males) and granulosa cells (in females), selectively inhibits FSH specifically, without affecting LH.
Applied Walkthrough
1
The hypothalamus releases GnRH in a pulsatile pattern, stimulating the anterior pituitary to release FSH and LH — which then act on the gonads to produce sex hormones.
2
For most of the reproductive cycle, these sex hormones feed back negatively, keeping GnRH, FSH, and LH within a normal range.
3
But at mid-cycle in a woman's menstrual cycle, rapidly rising estrogen levels flip this relationship: instead of suppressing the axis, high estrogen now stimulates a surge of LH — triggering ovulation, the one major exception to the usual negative feedback pattern.
4
A patient being treated for prostate cancer might receive a continuous (non-pulsatile) GnRH agonist specifically to exploit the paradoxical suppression this causes — deliberately shutting down testosterone production by disrupting the normal pulsatile signaling pattern the axis requires.
Exam Application
Exams test whether you understand why GnRH must be released in pulses (continuous GnRH actually suppresses the axis), whether you can distinguish FSH's and LH's separate roles, and whether you know the mid-cycle positive feedback exception that triggers ovulation.
⚠ Common Trap
The most common trap is assuming sex hormones always suppress the HPG axis — while this is true most of the time, the mid-cycle estrogen surge in females is a critical exception, switching to positive feedback and triggering the LH surge that causes ovulation.
✓ Quick Self-Check
1. Why must GnRH be released in pulses rather than continuously?
Because continuous GnRH paradoxically suppresses the axis by downregulating receptors, while pulsatile release is required for normal function.
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2. What does FSH do in females versus males?
Drives follicle development in females; drives spermatogenesis and Sertoli cell support in males.
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3. What does LH do in females versus males?
Triggers ovulation and supports the corpus luteum in females; stimulates testosterone production via Leydig cells in males.
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4. When does the HPG axis switch from negative to positive feedback, and what does it cause?
Mid-cycle in females, when high estrogen triggers positive feedback, causing the LH surge and ovulation.
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5. What does inhibin selectively inhibit?
FSH specifically, without affecting LH.
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