Hypothalamicpituitarygonadal

The hypothalamic-pituitary-gonadal axis (HPG axis) is the hormonal system that regulates reproduction and sexual development in humans and many other vertebrates. It involves the hypothalamus releasing gonadotropin-releasing hormone (GnRH) in pulsatile bursts, which stimulates the anterior pituitary to secrete the gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH and FSH act on the gonads to drive gonadal steroid production and gametogenesis.

In males, LH stimulates Leydig cells to produce testosterone, while FSH acts on Sertoli cells to support spermatogenesis; inhibin B provides negative feedback on FSH. In females, FSH promotes ovarian follicle development, and LH triggers ovulation and luteinization; the ovaries produce estrogen and progesterone with the corpus luteum. The axis is regulated by feedback loops: sex steroids feed back to the hypothalamus and pituitary, and GnRH pulse frequency can influence the relative secretion of LH and FSH; higher GnRH pulse frequencies favor LH release, and estrogen can exert positive feedback during the mid-cycle.

During puberty, increasing GnRH pulsatility initiates maturation of the axis, leading to gonadal function and the development of secondary sexual characteristics. Clinically, disruption of the axis can cause hypogonadism or infertility, due to hypothalamic, pituitary, or gonadal pathology. Genetic conditions such as Kallmann syndrome involve GnRH deficiencies and often anosmia. Treatments may include hormone replacement therapy or assisted reproductive technologies, depending on the underlying cause.