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Gonadotrophin releasing hormone plays a central role in the menstrual cycle. It is not so much absolute level of GnRH, but the frequency combined with the amount of GnRH release that exerts the effects in menstruation.
The frequency of pulsed release is increased by increased levels of oestrogen; this is what occurs towards the end of the follicular phase of the menstrual cycle as the follicle matures and produces more oestrogens.
The frequency is decreased by progesterone and testosterone; the action of progesterone in the secretory phase of menstruation is to decrease GnRH levels, until progesterone levels fall at the end of the cycle.