Hormone replacement therapy (HRT) provides women with, or replaces, hormones that their ovaries stop making. The ovaries make estrogen and progesterone as part of the menstrual cycle. The ovaries also make androgens, including testosterone. Hormone levels usually start to change during perimenopause, the years just before menopause. In perimenopause, many women begin to have symptoms such as hot flashes, vaginal dryness, irregular periods, and insomnia. The reason for the symptoms is usually fluctuating and declining hormone levels.
These hormonal changes are due to aging ovaries that are losing their ability to produce eggs and hormones. Women may have widely varying estrogen levels during their monthly cycles as the ovaries keep trying to produce eggs. Often there will be cycles in which no ovulation occurs (called anovulatory cycles) and no progesterone is produced. This can lead to irregular cycles with heavy or abnormal bleeding as a woman approaches menopause.
At menopause, the ovaries stop producing and releasing eggs (ovulating) each month, and monthly periods stop completely. The ovaries are no longer producing estradiol and progesterone (although they continue to produce androgens). Menopause does not mean, however, that you have no estrogen in your body anymore. Women with more body fat usually have more estrogen than thinner women do, because estrogen is aromatized in fatty tissue.
After menopause, there is much less estrogen and testosterone in the body than before menopause, and very little progesterone. This drop in hormone levels can have various effects, including the familiar symptoms of menopause such as hot flashes, night sweats, vaginal dryness, and loss of libido.
A woman who has a surgical menopause, in which her ovaries are removed (usually along with her uterus and fallopian tubes), will have a much more sudden drop in all sex hormone levels than a woman going through natural menopause. This sudden loss of hormones can cause severe symptoms and health problems, especially in younger women, unless adequate hormone replacement is provided.