Effects of tamoxifen on endometrial estrogen and progesterone receptor concentrations in women with fibrocystic disease of the breast

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Abstract
Endocrine changes were determined after a 3-week cycle of tamoxifen treatment in 11 regularly cycling women with clinical and radiological evidence of fibrocystic disease of the breast. Blood and endometrial samples were obtained during the luteal phase prior to and at the end of treatment. Tamoxifen treatment (20 mg/day orally for 3 weeks), produced a significant increase in plasma estradiol (p = 0.0018) without simultaneous changes in plasma luteinizing hormone, follicle stimulating hormone, prolactin or progesterone. Tamoxifen treatment significantly reduced endometrial estrogen receptor levels compared to the control cycle (p = 0.0018) while endometrial progesterone receptor levels remained unchanged. Endometrial histological studies showed secretory transformation in both the control cycle and after tamoxifen treatment. The reduction in endometrial estrogen receptor concentrations would suggest a tamoxifen-induced effect or a down-regulatory mechanism to protect target tissues from high estradiol levels. These changes were not associated with alterations in either plasma progesterone or endometrial progesterone receptor concentrations. The tamoxifen-induced changes did not produce any interference in the glandular secretory response of the endometrium.