Effect of norethindrone acetate released from a single silastic implant on serum FSH, LH, progesterone and estradiol-17beta of women during first eight months of treatment

4 healthy women of proven fertility and normal menstrual cycles were implanted with a single silastic implant D, filled with norethindrone acetate (ENTA) in order to study the effect of continuously released low levels of ENTA (150 mcg/day) on serum levels of follicle stimulating hormone, luteinizing hormone, estradiol-17beta, and progesterone and to assess the site of action. Steroids were measured by radioimmunoassay before the implant and 2-4 cycles during the first 8 months of treatment. Values of steroids prior to implant indicate that all subjects were ovulatory before treatment. During the 1st few months of use, values indicate that ovulation continued unsuppressed. Later cycles demonstrated hormonal disturbances with low serum progesterone during the luteal phase in most of the treatment cycles when compared with controls. Results indicate a gradually increasing suppression on the pituitary-gonadal function.