hypothalamic-pituitary-gonadal axis

The HPG axis is the chain of command that runs reproduction. Like a corporate hierarchy, orders flow from the brain at the top down to the gonads, and reports flow back up, keeping sex-hormone output and fertility coordinated rather than chaotic.

The axis has three layers. The hypothalamus releases GnRH in pulses; GnRH tells the anterior pituitary to release the gonadotropins LH and FSH; and these act on the gonads (testes or ovaries). In men, LH drives Leydig cells to make testosterone, while FSH supports Sertoli cells and sperm production. The gonadal hormones then feed back negatively on the hypothalamus and pituitary, with testosterone and estradiol restraining GnRH and LH, and inhibin B selectively restraining FSH.

A crucial feature is that GnRH must be released in pulses, not continuously; steady GnRH actually shuts the system down. This quirk is exploited medically: long-acting GnRH analogs given continuously suppress the whole axis to lower sex hormones, useful in prostate cancer or precocious puberty. Reading where the axis is broken (low hormones with low LH points to the brain or pituitary; low hormones with high LH points to the gonad) is the basic logic of diagnosing reproductive disorders.