Hormonal Control of Human Reproduction

Core Concepts

The HPG axis and its players

The relay runs top-down: hypothalamus → GnRH → anterior pituitary → FSH and LH → gonads → sex steroids (testosterone in males; estrogen and progesterone in females). GnRH is released in pulses, and the pituitary responds to pulse frequency. Each level reports back: gonadal hormones feed back to the hypothalamus and pituitary, mostly suppressing further release — negative feedback — keeping hormone levels in a narrow operating range. The male system is the simplest case: steady GnRH pulses drive steady FSH/LH, which drive steady testosterone and continuous sperm production.

Male hormonal control: a steady loop

In males, LH stimulates Leydig cells to secrete testosterone; testosterone is needed for spermatogenesis (working with FSH on Sertoli cells) and for male secondary sex characteristics. FSH acts on Sertoli cells to support sperm maturation. Feedback: testosterone suppresses GnRH and LH (and to a lesser degree FSH), while inhibin — a peptide made by Sertoli cells — selectively suppresses FSH. The result is a stable, continuous system: constant production, constant feedback. If testosterone is supplied externally (as in some athletes' use), the axis shuts down — GnRH, LH, and FSH fall, and the testes shrink and stop producing sperm (a well-documented feedback effect; educational description, not advice).

The ovarian cycle: three phases and one surge

The female system cycles. The ovarian cycle has three phases:

1. Follicular phase: rising FSH stimulates a follicle to mature; granulosa cells in the follicle produce rising estrogen. Early on, estrogen exerts negative feedback on the pituitary, keeping FSH/LH low.
2. Ovulation: as estrogen crosses a threshold, the feedback flips sign — high estrogen now stimulates the pituitary (positive feedback), producing a massive LH surge that triggers the release of the secondary oocyte about 24–36 hours later (commonly taught timing). This low-inhibits/high-stimulates switch is the most tested concept in this topic.
3. Luteal phase: the ruptured follicle becomes the corpus luteum, which secretes progesterone (and some estrogen). Progesterone and estrogen now exert negative feedback, keeping FSH and LH low so no new follicle matures; without fertilization the corpus luteum degenerates after about two weeks, hormones crash, and the cycle restarts.

The uterine cycle: preparing the lining

In parallel, the uterine (menstrual) cycle prepares the endometrium. Menses (days 1–5 of the classic 28-day average): the endometrial lining sheds because progesterone and estrogen have fallen. Proliferative phase: rising estrogen rebuilds the lining — new blood vessels and glands. Secretory phase: after ovulation, progesterone from the corpus luteum makes the lining thick, vascular, and glandular — ready to receive an embryo. If no embryo implants, the corpus luteum dies, hormones drop, and the lining sheds again. The two cycles are synchronized by the same hormones: estrogen drives the proliferative phase; progesterone drives the secretory phase.

Pregnancy and menopause: the axis's two endpoints

If fertilization occurs, the embryo secretes human chorionic gonadotropin (hCG), which acts like LH and maintains the corpus luteum, keeping progesterone high so the lining is not shed — the basis of pregnancy tests. Eventually the placenta takes over hormone production. At the other end of life, menopause occurs when the finite pool of follicles is depleted: estrogen falls and cycles stop. Both endpoints are direct consequences of the same axis — rescue it with hCG and the cycle stops; run out of follicles and it stops.

Common Confusions