There are two primary types of hormone replacement therapy (HRT):

• Estrogen. Comparable to the hormone testosterone in men, estrogen is a major female hormone that affects both the brain and the body. During puberty, it is primarily responsible for secondary sexual characteristics (e.g., breast development, pubic hair, broadening of the hips). It is also chiefly involved in the internal physical changes that allow a woman to carry a pregnancy.
  
  There are three types of estrogen:
  
• • Estrone (E1). Produced by the ovaries.
    
    
  • Estradiol (E2). Produced by the ovaries, it is the most abundant estrogen.
    
    
  • Estriol (E3). Produced by the placenta during pregnancy.
    
    

    
    

  Normal concentrations of estrone are produced in body fat. Women who are exceptionally thin will not have monthly periods because there is not enough fat to produce estrone. Estrone and estradiol are the two types of estrogen produced in the ovaries – the two organs leading to a woman’s uterus (womb) that contain eggs (ova). The ovaries begin producing estrogen around the age of eight or nine years and gradually stop producing it around the time of menopause (12 months after the last menstrual period). Not only is estrogen responsible for processes related to the female reproductive organs, but it is also present in hundreds of other tissues in the female body.
  
  Estrogen treatment is given to women who no longer have a uterus. It is made synthetically from plants and other sources, or made from the urine of pregnant horses. Estrogen may be taken by pill, skin patch, vaginal cream, vaginal ring, suppository or injection. The method of delivery may depend on the nature of the symptoms. For example, a cream may ease vaginal dryness, whereas a pill or skin patch may help reduce hot flashes.
  
  For many years, menopausal and post-menopausal women were given estrogen to help reduce symptoms associated with menopause. Known as estrogen replacement therapy (ERT), this treatment was used because it was recognized that estrogen is the main sex hormone that produces menopause-related changes.
  
  However, when ERT is taken alone, it can lead to abnormal thickening of the uterine lining (uterine hyperplasia), which significantly increases the risk of uterine cancer and endometrial cancer (cancer in the lining of the uterus). This is because a postmenopausal woman’s ovaries not only stop producing estrogen, but they also stop producing another sex hormone called progesterone, which protects the uterus. Because postmenopausal women do not have enough progesterone to take ERT without increasing the risk of uterine and/or endometrial cancer, ERT is now recommended only for women who have had a hysterectomy (the surgical removal of the uterus).
  
  
• Estrogen and progesterone. For women who still have a uterus, estrogen is usually taken with a form of progesterone. The form of progesterone most often prescribed is chemically produced: a synthetic progestin, almost always given in pill form. This type of HRT can be administered according to different schedules, including:
  
• • Cyclical estrogen and progestin. This involves taking estrogen daily and adding progestin between 12 and 14 days every month. During five or six days each month, the patient takes no hormones. The regimen is often used in women who are still having menstrual periods at the time they begin HRT. Most women will experience a light period during the days when they are not taking hormones.
    
    
  • Daily estrogen and progestin. Also known as “continuous combined HRT,” it involves taking both estrogen and progestin every day. Women who have not had a period for at least six to 12 months often use this regimen. Many will initially experience episodes of irregular spotting or bleeding that usually ends within six to eight months.