Final answer:
Progesterone is important in hormone therapy as it balances estrogen's effects, reducing the risk of endometrial cancer by preventing hyperplasia of the endometrium.
Step-by-step explanation:
Progesterone plays a critical role in counterbalancing the effects of estrogen during hormone therapy, thereby decreasing the risk of endometrial cancer. During the menopausal transition, often called peri-menopause, estrogen levels remain steady but the production of progesterone by the corpus luteum diminishes. This decline in progesterone can lead to excess proliferation of the endometrial lining, a condition known as hyperplasia, which subsequently increases the risk of developing endometrial cancer. In hormone therapy, progesterone is added to estrogen to prevent this excessive growth of the endometrial tissue, reducing the likelihood of hyperplasia and consequent malignancy.