hypothalamic-pituitary-gonadal axis
Summary
The HPG axis is a hormonal feedback system regulating reproduction and sexual development. GnRH from the hypothalamus stimulates pituitary release of LH and FSH, which act on the gonads to produce sex steroids and gametes. This axis is central to puberty, menstrual cycling, spermatogenesis, and is a key target in reproductive endocrinology and pharmacology.
Detail
The hypothalamic-pituitary-gonadal (HPG) axis begins with pulsatile secretion of gonadotropin-releasing hormone (GnRH) from hypothalamic neurons into the hypophyseal portal system. GnRH stimulates gonadotroph cells in the anterior pituitary to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Pulsatile GnRH secretion is critical—continuous administration paradoxically suppresses LH/FSH release (used therapeutically in GnRH agonists like leuprolide for prostate cancer, endometriosis, and precocious puberty).
In males: LH stimulates Leydig cells to produce testosterone; FSH acts on Sertoli cells to support spermatogenesis and produce inhibin B, which provides negative feedback on FSH. Testosterone (and its conversion to estradiol via aromatase) provides negative feedback on both hypothalamic GnRH and pituitary LH/FSH.
In females: FSH stimulates growth of ovarian follicles and estrogen production by granulosa cells; LH stimulates theca cells to produce androgens (converted to estrogen by granulosa cells) and triggers ovulation via the LH surge. Estrogen exerts negative feedback at low/moderate levels but positive feedback at high sustained levels (mid-cycle), triggering the LH surge necessary for ovulation. Progesterone from the corpus luteum reinforces negative feedback in the luteal phase. Inhibin B (early follicular) and inhibin A (luteal phase) selectively suppress FSH.
Clinical relevance: Disruption of this axis causes hypogonadism, which can be classified as hypogonadotropic (hypothalamic/pituitary failure, e.g., Kallmann syndrome, characterized by anosmia and low GnRH) or hypergonadotropic (primary gonadal failure, e.g., Klinefelter syndrome, Turner syndrome, with elevated LH/FSH due to loss of negative feedback). Functional hypothalamic amenorrhea (stress, low body weight, excessive exercise) suppresses GnRH pulsatility. Polycystic ovary syndrome (PCOS) involves increased LH:FSH ratio and androgen excess. Pharmacologic manipulation includes GnRH agonists/antagonists, clomiphene (SERM that blocks estrogen negative feedback to boost gonadotropins in infertility), and hormone replacement therapies.
Sources
- Guyton and Hall Textbook of Medical Physiology
- Harrison's Principles of Internal Medicine
- First Aid for the USMLE Step 1
- Kaplan USMLE Step 1 Physiology
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