The Role of Estrogen in Luteinizing Hormone Surge and Ovulation

Sine the common understanding of ovulation in a natural cycle suggests that a sustained, elevated estradiol level is required to trigger the LH surge, administration of letrozole throughout the cycle should lower estradiol levels and prevent the LH surge from occurring. In this study, we sought to determine if the LH surge, ovulation and luteinization occurs in spite of low estradiol levels by daily administration of letrozole in a group of normal ovulatory volunteers in a prospective study.

After IRB approval and informed consent were obtained, ten willing volunteers that met inclusion criteria (no hormonal contraception within 3 months, regular menstrual cycles 26 - 30 days, normal thyroid function and normal prolactin, and no pregnancy currently or within 3 months) were monitored for one month without treatment for evaluation of normal ovulation.

Natural control cycle The subjects used home urine LH tests (Clearblue® Easy, SPD Swiss Precision Diagnostics, Switzerland) on days 10-18 to monitor for the LH surge in both the initial natural cycle and the letrozole cycle. Blood was drawn every other day starting on day 12 of the cycle through day 22 to measure estradiol and progesterone levels, and follicular development was monitored using transvaginal ultrasound on cycle day 12-14.

Letrozole cycle In the next cycle, all ten subjects were administered oral letrozole 5 mg daily (Femara®, Novartis Pharmaceuticals Corporation, East Hanover, NJ ) starting on cycle day 1-3 and continuing through the completion of the study (cycle day 22). Once again, serum estradiol and progesterone levels were measured every other day on days 12-22. The development of the ovarian follicles was monitored by transvaginal ultrasound once in each cycle between days 12-14, and LH surge was monitored with home urine ovulation tests on days 10-18. Table 1 illustrates protocols for both the natural control cycle and the letrozole study cycle.

The primary outcome, assessment of ovulation in letrozole cycles, was determined by the presence or absence of progesterone elevation (>1.5 ng/mL) and the presence or absence of a positive urinary LH test. The bioequivalence evaluation of two cycles (before and after letrozole administration) was based on pharmacokinetic parameters such as area under the serum concentration-time curve (AUC), the peak serum concentration (Cmax) and the time of peak serum concentration (Tmax). Cmax and Tmax were determined by visual inspection from each volunteer's serum concentration-time curve for estradiol and progesterone. AUC was calculated by the linear trapezoidal method from day 12 through day 22 in both the initial natural cycle and the letrozole cycle.