LOW-DOSE TRANSDERMAL ESTRADIOL SUPPRESSES GONADOTROPIN-SECRETION IN BREAST-FEEDING WOMEN

The object of the study was to investigate the effect on gonadotrophin secretion of a small increase in oestradiol concentration, A total of 13 fully breast-feeding women (12 weeks post-partum) underwent serial blood sampling at 10 mill intervals for 12 h on 2 different days; day 1 untreated and day 5 after 3 days of treatment with transcutaneous o estradiol (100 mu g/day), On both days bolus gonadotrophin-releasing h ormone (GnRH; 10 mu g i,v,) was given after a 10 h baseline period, In six of the subjects, a naloxone infusion was administered during the second study day, Application of transdermal oestradiol raised the oes tradiol concentration within the normal follicular phase range, The me an luteinizing hormone (LH) concentration on day 5 was found to be sig nificantly lower than that on day 1 (P < 0.05). The LH response to GnR H was, however, significantly higher on day 5 than day 1 (P < 0.001), The mean follicle stimulating hormone (FSH) concentration on day 5 was also significantly lower than that on day 1 (P < 0.01), while the pea k concentration after GnRH was unchanged, When the opioid antagonist n aloxone was infused after oestradiol treatment, the subjects with low pre-study oestradiol concentrations exhibited no effect on LH concentr ation, while in the subjects with higher oestradiol concentrations the LH concentration was increased. It was concluded that the administrat ion of small doses of oestradiol caused a significant fall in gonadotr ophin concentration in breast-feeding women, This indicates a heighten ed sensitivity to the negative feedback effect of oestradiol on GnRH r elease from the hypothalamus, since the pituitary response to GnRH was unaltered. Further studies are required to investigate the possibilit y that these doses of oestrogen may have some clinical value in inhibi ting ovulation after delivery.