HPG Axis — Hypothalamic–Pituitary–Gonadal

The hormonal cascade that governs reproduction in both sexes. The hypothalamus releases a pulsing signal, the pituitary gland translates it into two travelling hormones, and the gonads — testes in males, ovaries in females — respond by making sex hormones and gametes.

Signaling chain

GnRH (gonadotropin-releasing hormone, from the hypothalamus, released in pulses roughly every 60–120 minutes) → LH and FSH (luteinizing hormone and follicle-stimulating hormone, from the anterior pituitary’s gonadotrope cells) → sex steroids and gametes from the gonads:

• Testes — LH drives Leydig cells to make testosterone; FSH drives Sertoli cells to support spermatogenesis.
• Ovaries — LH and FSH orchestrate the menstrual cycle: follicular growth, the mid-cycle LH surge that triggers ovulation, and corpus-luteum progesterone.

Function

Reproduction is the obvious one — fertility, gametes, the menstrual cycle, pregnancy. But the axis also governs sexual development across the lifespan: the formation of male anatomy in the fetus, a brief reawakening just after birth, the long quiet of childhood, the dramatic activation of puberty, and the gradual senescence of later life. The sex hormones it produces shape secondary sex characteristics, bone density, muscle mass, body composition, libido, and aspects of mood and cognition that reach far beyond the reproductive organs themselves.

Going deeper

• Pulsatility — the defining feature of the axis: GnRH must be released in bursts, not continuously. How the pulse generator works and why frequency matters.
• Feedback architecture — the three feedback loops (testosterone, estradiol, inhibin B) that keep the axis self-correcting, and why their distinct signatures are diagnostically powerful.
• Inputs to the axis — the signals from outside the axis that turn it up or down: energy status, stress, prolactin, thyroid hormone, opioids.
• Across the lifespan — how the axis behaves from fetal life through minipuberty, the childhood quiet, puberty, adulthood, and senescence.
• Deviations — what goes wrong at each level, and how the lab pattern points to the lesion: central versus primary hypogonadism, congenital and acquired causes, functional suppression.

Key hormones

Hormone-specific deep dives for the molecules that appear repeatedly across these pages:

• Inhibin B — the Sertoli-cell hormone that closes the FSH–Sertoli feedback loop; one of the most useful single labs in male reproductive medicine.
• AMH — Anti-Müllerian Hormone; wires the male anatomy in utero by causing the Müllerian duct system to regress, and afterwards tracks Sertoli reserve (males) and ovarian reserve (females).

Key idea

The HPG axis is a stable oscillator: a pulsing brain signal, amplified through the pituitary, kept in check by feedback from the gonads. The rhythm matters as much as the magnitude — continuous GnRH shuts the axis down rather than driving it harder, which is why this is one of the few endocrine systems where timing is part of the message.

Key labs

LH, FSH, testosterone (with attention to morning timing), estradiol, progesterone, prolactin, SHBG, inhibin B, AMH.