Hypothalamic-Pituitary-Gonadal (HPG) Axis

The HPG axis runs from hypothalamic GnRH neurons, gated by kisspeptin input, through pituitary gonadotropes releasing LH and FSH, to the gonads, which produce testosterone, estradiol and progesterone along with inhibin B. Feedback closes the loop at both upper levels and is negative nearly everywhere: sex steroid restrains GnRH and gonadotropin output, while inhibin B selectively restrains FSH. The single exception is the preovulatory switch, where sustained high estradiol flips the same feedback to positive and produces the LH surge.

The axis is transiently active in the first months of life, the phase called minipuberty, falls quiet through childhood, and reawakens at puberty with GnRH pulses that appear first during sleep. The gatekeeper is kisspeptin signalling, established when loss-of-function mutations in its receptor were reported in familial hypogonadotropic hypogonadism in 2003. Clinically the axis is read in one move: low testosterone with high LH and FSH localises the problem to the gonad, low testosterone with low or normal gonadotropins to something above it.

That localisation determines what an intervention can do. hCG or FSH drive the gonad directly and work only if the gonad is capable; clomiphene and aromatase inhibitors act on feedback and require a functioning pituitary; pulsatile GnRH acts at the pituitary and requires functioning gonadotropes; exogenous testosterone bypasses the loop and suppresses it.

Two misreadings dominate. The first treats suppression as reversible on a fixed timetable, often under the abbreviation HPTA, when recovery data are observational, highly variable, and confounded by whatever else was taken. The second infers the state of the axis from one blood draw: LH is pulsatile and testosterone both pulsatile and diurnal, so a single pair carries more intrinsic variation than the differences read from them.