Irregulat menstrual cycles are associated with high levels of prolactine (hyperprolactinaemia). Hyperprolactinaemia deregulates systems and processes affected by the pituitary and gonadal hormones.
The great response of prolactin in women of a reproductive age, who are not nursing or pregnant, leads to the inhibition of the normal pulsatile secretion of gonadotropin-releasing hormone (GnRH) of the hypothalamus.
These, not so frequent, pulses of GnRH result in regular menses, on the one hand, but impaired follicular growth on the other. Greater impairment of pulsatile GnRH secretion leads to an anovulatory (ovulation does not occur) stage with menses being too frequent, too heavy, or infrequent.
Further restraining of pulsatile GnRH secretion provokes deficient secretion of luteinizing hormone and follicle stimulating hormone (FSH), in amounts not adequate to induce a proper ovarian response. The ovulation does not occur and no egg will be relised.
That provokes a hypoestrogenized (low estrogen) amenorrheic and anovulatory cycle and side reactions of estrogen deficiency is comparable to what occurs during menopause or infertility.