What Is the HPG Axis? Hypothalamic-Pituitary-Gonadal Axis Explained | QSC
QSC RESEARCH GUIDE
What Is the HPG Axis? Hypothalamic-Pituitary-Gonadal Axis Explained
The hypothalamic-pituitary-gonadal (HPG) axis is the hormonal cascade that controls reproductive function and gonadal steroid production in both sexes. Understanding the HPG axis is fundamental to research with GnRH, kisspeptin, gonadotropins (LH/FSH), HCG, HMG, and androgen compounds. This guide covers the full HPG cascade, feedback mechanisms, and research tools.
TRT model: testosterone administration → LH/FSH suppression
Long-loop positive (female)
High oestradiol on hypothalamus/pituitary
↑GnRH/LH surge → ovulation trigger
LH surge model: high oestradiol → positive feedback
Short-loop negative
LH on pituitary gonadotrophs
Direct autocrine/paracrine LH suppression
Less studied — primarily clinical relevance
Inhibin B negative
Inhibin B (Sertoli/granulosa) on pituitary
Selective FSH suppression without LH effect
FSH-specific research — isolating FSH feedback from LH
Why GnRH pulsatility is essential
Continuous GnRH exposure desensitises GnRH receptors on gonadotrophs — paradoxically suppressing LH and FSH. This is the mechanism of GnRH agonist therapy (leuprolide) for prostate cancer and endometriosis. Only pulsatile GnRH (mimicking the ~90-minute endogenous pulse) maintains HPG axis stimulation. Kisspeptin-10 bolus produces a single GnRH-equivalent pulse — enabling timed HPG stimulation research.
ITT maintenance (HCG), spermatogenesis preservation (HMG)
LH surge model (female)
Sequential E2 priming → E2 bolus → LH surge
Tests kisspeptin/GnRH amplification of LH surge
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Frequently Asked Questions
What is the HPG axis?
The HPG (hypothalamic-pituitary-gonadal) axis is the hormonal cascade controlling reproductive function: GnRH (hypothalamus) → LH + FSH (pituitary) → gonadal steroids (testes: testosterone, ovaries: oestradiol/progesterone). Upstream regulation by kisspeptin neurons drives GnRH pulsatility — the master switch for HPG activation.
How does exogenous testosterone suppress the HPG axis?
Testosterone (and its aromatised product oestradiol) act at the hypothalamus and pituitary to suppress GnRH pulse frequency and gonadotroph LH/FSH secretion via negative feedback. At therapeutic/supraphysiological doses, this completely suppresses LH → Leydig cells lose stimulation → intratesticular testosterone falls → spermatogenesis is impaired.
What is the difference between LH and FSH in male reproduction?
LH acts on Leydig cells in the testes to stimulate testosterone synthesis. FSH acts on Sertoli cells to support spermatogenesis (ABP production, blood-testis barrier, germ cell nurturing). Both are required for complete fertility: HCG (LH receptor agonist) restores testosterone but cannot fully restore spermatogenesis without FSH.
What is kisspeptin’s role in the HPG axis?
Kisspeptin-secreting KNDy neurons in the arcuate nucleus are the GnRH pulse generator — their synchronised activity (driven by neurokinin B) drives pulsatile GnRH release. Kisspeptin-10 activates KISS1R on GnRH neurons, triggering a GnRH pulse → LH release within 5-15 minutes. Loss-of-function KISS1R mutations cause hypogonadotropic hypogonadism.
