Antiestrogen action of progesterone in breast tissue.

In normal breast, estrogen stimulates growth of the ductal system, while lobular development depends on progesterone. Thus, estrogen and progesterone, when secreted in an adequate balance, permit the complete and proper development of the mammary gland. Progesterone may also have an antagonistic activity against estradiol, mediated through a decrease in the replenishment of the estrogen receptor, and also through increased 17 beta-hydroxysteroid dehydrogenase which leads to accelerated metabolism of estradiol to estrone in the target organ. Thus, it can be inferred that long periods of luteal phase defect leading to an unopposed estrogen effect on the breast might promote breast carcinogenesis.