Urinary N-telopeptides to monitor bone resorption while on GnRH agonist therapy

Objective: To assess the utility of urinary cross-linked N-telopeptides in monitoring bone resorption and predicting bone loss during GnRH agonist administration. Methods: Ninety patients who were prescribed GnRH agonist therapy for 3-6 months for treatment of endometriosis, leiomyomas, or other gynecologic disorders participated in this prospective multicenter study. N-telopeptides, serum estradiol (E2), and bone mineral density were monitored before, during, and up to 3 months after the course of GnRH agonist therapy. Results: N-telopeptide levels increased significantly throughout GnRH agonist therapy and returned to baseline levels by 3 months after treatment was completed. A significant negative correlation was seen between N-telopeptide and E2 measurements after 3 months (r = -0.23, P < .05), 4 months (r = -0.32, P < .05), and 5 months (r = -0.41, P < .005) of GnRH agonist therapy. The percent change in bone mineral density at L1-L4 at 6 months of GnRH agonist treatment correlated inversely with the percent change in N-telopeptides from baseline to 2, 3, 4, and 5 months of treatment; the percent change in bone mineral density at the femoral neck at 6 months correlated inversely with the percent change of N-telopeptides from baseline to month 4. Conclusions: Urinary N-telopeptide determinations provide a quantitative measure of bone resorption, due to GnRH agonist-induced hypoestrogenism. Increases in resorption as measured by N-telopeptides parallel decreases in E2 levels. Increases in N-telopeptides on GnRH agonist therapy may provide a tool to predict decreases in bone mineral density.