HPO axis
The hypothalamic-pituitary-ovarian axis, the signalling chain between brain and ovary that runs the menstrual cycle. Disrupted in PMOS.
The HPO axis is the communication chain that runs the menstrual cycle. The hypothalamus in the brain releases GnRH in pulses; the pituitary responds by releasing LH and FSH; and the ovary responds to those by maturing follicles and producing hormones. Hormones from the ovary then feed back to the brain to adjust the next round. It is a loop.
In PMOS, this loop is disrupted. The GnRH pulses tend to run fast, which skews the pituitary toward more LH relative to FSH. The ovary, under that signal and with elevated insulin, produces more androgens than oestrogen, and follicle maturation stalls. The androgens then blunt the feedback that should slow the pulses down, so the loop stays stuck.
This is why cycle irregularity in PMOS is not only an ovarian problem. It is a breakdown in the conversation between the brain and the ovary.