The addition of progesterone to estrogen therapy in postmenopausal women primarily reduces the risk of which condition?

The addition of progesterone to estrogen therapy in postmenopausal women is primarily aimed at reducing the risk of endometrial cancer. When estrogen is administered without the balancing effect of progesterone, there is an increased stimulation of the endometrium (the lining of the uterus) which can lead to hyperplasia and, subsequently, an increased risk of endometrial cancer. Progesterone works to counteract the effects of estrogen on the endometrial lining by inducing a more regulated growth pattern and shedding of the lining that occurs during the menstrual cycle. Therefore, integrating progesterone into estrogen therapy is critical for women who still have a uterus to mitigate the risk of developing endometrial cancer due to unopposed estrogen exposure.

In contrast, while the other conditions mentioned may be associated with hormone therapy, they are not primarily addressed through the addition of progesterone in the same manner as endometrial cancer. For example, while there might be some discussion regarding breast cancer risk in relation to hormone therapy, the specific protective mechanism of progesterone pertains more directly to the endometrium.