Tamoxifen-induced ovarian hyperstimulation during premenopausal hormonal therapy for breast cancer in Japanese women

PURPOSE Tamoxifen is an anti-estrogenic drug that is widely used for endocrine-dependent breast cancer as adjuvant hormonal therapy, and its use has been reported to be frequently associated with high levels of serum estradiol. Since the population of premenopausal women receiving tamoxifen therapy is growing in Japan, we retrospectively analyzed the incidence of ovarian hyperstimulation by tamoxifen therapy in Japanese women. METHODS Eleven patients who received surgical therapy for endocrine-dependent breast cancer and showed high values of serum estradiol during post-operative tamoxifen therapy were recruited in this study and evaluated by examining the serum concentration of follicular stimulating hormone (FSH) and follicular development. RESULTS The mean age, serum concentrations of estradiol and FSH, and follicular diameter were 41.3 years old, 1015.8 pg/mL, 11.8 mIU/mL, and 3.47 cm, respectively. In 6 cases, multiple follicular development was observed, while the other cases showed single follicular development with a mean serum estradiol level of 848.6 pg/mL and follicular diameter of 4.46 cm. There was no significant difference in age or FSH concentration between the two groups. The mean periods from the start of the single administration of tamoxifen to the initial detection of a high estradiol concentration was 716.5 days. CONCLUSIONS These findings indicate that tamoxifen could stimulate the ovarian function even after 2-year treatment. Since single and multiple follicular developments with large sizes were observed, dual mechanisms through the inhibition of both negative and positive feedback to the hypothalamic-pituitary-axis can be proposed to explain the adverse effects of tamoxifen on ovarian function.