Estrogen replacement therapy refers to the administration of estrogen to women who have reduced levels of estrogen, especially after menopause or surgical removal of the ovaries.

Taking estrogen alone can cause a high risk of uterine cancer. As a result, estrogen is almost always proscribed together with progestin (a regimen called hormone replacement therapy or HRT), which is associated with a reduced risk of causing cancer. However, if the uterus has been removed (a process called hysterectomy), progestin is not required.

Further Reading:

Article : "ESTROGEN AND ALZHEIMER'S"