Is the contraceptive effect of 300 micrograms of norethisterone mainly peripheral or central?

The effect of norethisterone (NET) on pituitary function was evaluated by measuring circulating follicle stimulating hormone (FSH) and luteinizing hormone (LH) during the mid follicular phase as well as before and after removal of the corpus luteum (CL) in 30 untreated women and 43 women treated with 300 micrograms of NET daily for three months. All untreated women exhibited significantly elevated FSH levels after removal of the CL irrespective of the stage of the luteal phase. In the NET-treated women, the levels of FSH were significantly raised in women with luteal activity but these levels were not influenced by the operative procedure in those women without luteal activity. The LH levels remained unchanged after removal of the CL in both untreated and NET-treated women. The results suggest that a low dose of gestagen exerts variable effects on pituitary function. The main contraceptive effect seems to be through direct interference with ovarian function.