Background: The endogenous LH surge is the result of the estrogen positive feedback effect. However, the factors that are responsible for the termination of LH surge are not known.

Objective: To investigate the mechanism that terminates the LH surge in women.

Subjects and methods: Eight normally cycling women (aged 42-48 yr) were investigated in two cycles, i.e. cycle-1 (control) and cycle-2. In cycle-2 total abdominal hysterectomy plus bilateral salpingooophorectomy was performed on day 3. In both cycles, estradiol was administered transdermally at the dose of 100 µg on day 3 and 150 µg on days 4 and 5. Blood samples were obtained every 12 h from days 3 to 5 and every 6 h thereafter until day 9.

Results: In both cycles, after suppression of gonadotrophins, the women displayed an endogenous LH surge. The time intervals between the commencement of estradiol treatment and the LH surge onset (73.5 ± 1.5 vs. 76.5 ± 2.5 h) and peak LH values (11.4 ± 1.9 vs. 12.4 ± 3.1 IU/l) were comparable in the two cycles (mean ± SEM). After peaking, LH values decreased gradually in cycle-1 while in cycle-2 they remained stable and were higher than the corresponding values in cycle-1 (P < 0.05). Before the LH surge onset, estradiol values showed in both cycles a preovulatory pattern of changes, but starting 24 h after the onset of the LH surge they were lower in cycle-2 (P < 0.05). Progesterone levels were similar in both cycles until the day of the LH surge onset, but in cycle-2 they declined thereafter and were lower than in cycle-1 (P < 0.05).

Conclusions: It is suggested that ovarian factors rather than exhaustion of pituitary reserves are important for termination of the endogenous LH surge during the normal menstrual cycle.