Effect of high dose exogenous oestrogen on midcycle luteinizing hormone surge in human spontaneous cycles

Induction of multiple follicular development in women results in supraphysiological serum oestradiol (E2) levels and marked reduction of the endogenous LH surge. To examine whether high serum E2 levels at midcycle can suppress gonadotrophin secretion during the LH surge, five normally ovulating women were given an intramuscular injection of 10 mg oestradiol benzoate 6-42 h before the expected endogenous LH surge. The results were compared with the preceding untreated control cycle. Serum E2 levels at the onset of the LH surge (mean +/- SEM) were significantly higher in the oestrogen-treated cycles (13,600 +/- 1963 pmol/l) than in the control cycles (1012 +/- 116 pmol/l) (P less than 0.001). The duration of the LH surge (64.8 +/- 2.2 h) and LH peak levels (136 +/- 9.6 mIU/ml) in the oestrogen treated cycles were similar to those in the control cycles (61.2 +/- 3.9 and 119 +/- 9.4 mIU/ml respectively) (mean +/- SEM). The cycle day on which the LH surge started and the size of the dominant follicle, as assessed by ultrasound, did not differ significantly between the two groups of cycles. These results show that in spontaneous cycles the endogenous LH surge is neither reduced nor amplified by the induction of supraphysiological serum E2 levels during the immediate pre-ovulatory stage.