Aromatase inhibition causes increased amplitude, but not frequency, of hypothalamic-pituitary output in normal women

Objective: To better understand the site and mode of action of aromatase inhibitors. Design: Prospective study. Setting: Academic research environment. Patient(s): Five eumenorrheic (without polycystic ovary syndrome), early follicular phase women with a normal body mass index (mean: 20.47 ± 0.68 kg/m ²), and 12 normal weight, midreproductive aged, early follicular phase women with a normal body mass index (mean: 20.8 ± 1.7 kg/m ²) as historical controls. Intervention(s): 2.5 mg letrozole daily for 7 days, with daily urine collection (first morning void), thrice weekly blood sampling, and 4 hours of blood sampling every 10 minutes. Main Outcome Measure(s): Serum luteinizing hormone (LH) measured by a well-characterized immunofluorometric assay with LH pulse characteristics compared between treated and control groups using t tests. Result(s): Mean LH and LH pulse amplitude more than doubled in the women who had taken letrozole compared with the controls, but the LH pulse frequency did not differ between the women taking letrozole and the controls. Conclusion(s): These results indicate that the release of negative feedback inhibition of estradiol on the hypothalamic-pituitary axis in normal women by aromatase inhibitors creates an amplitude-related increase in endogenous hypothalamic-pituitary drive. The finding that the mean LH and LH pulse amplitude, but not the frequency, increased after letrozole suggests a possible pituitary site of action.