Objective: To assess the effects of an oral soy isoflavone extract (Phytosoya) on endometrium (evaluated by biopsy and ultrasonography) in postmenopausal women treated for 12 months.
Design: A total of 395 postmenopausal women were included in this international prospective, open-label study. The women were treated for 12 months with a specific standardized soy isoflavone extract (total of 70 mg/d). Endometrial biopsy and transvaginal ultrasonography were performed before and after 12 months of treatment according to European guidelines.
Results: A total of 301 assessable biopsy specimens were obtained from women treated for 12 months; the results were 99.67% atrophic/inactive endometrium and 0.33% proliferative endometrium. No case of hyperplasia or carcinoma was diagnosed, demonstrating the endometrial safety of this extract (point estimate: 0.0; upper limit of 95% CI: 0.012). Endometrial thickness did not show any increase after 12 months of treatment (2.2 mm at inclusion and 2.12 mm at the end of the study). Only eight women reported some kind of bleeding as an adverse event during the study.
Conclusions: These results of endometrial biopsy and endometrial thickness suggest that daily administration of 70 mg of a specific and standardized isoflavone extract for 12 months does not stimulate the endometrium.