Estrogen is a steroid hormone that signals to a number of tissues throughout the body. Estrogen is secreted primarily from the ovaries in women, and is also produced in lower levels by the adrenal glands and fat tissue. Estrogen travels throughout the bloodstream to signal to estrogen-responsive cells. These cells contain a protein called the estrogen receptor that can bind to circulating estrogen and produce an appropriate response in the cell. There are a number of forms of estrogen, each of which works differently in its signaling process.
Estrone
One form of estrogen is estrone, which is normally produced by the ovaries in the absence of pregnancy. Estrone is one of the regulators of the menstrual cycle, along with other forms of estrogen. Estrone levels can be used to help predict the time of ovulation, according to a study published in the "Journal of Women's Health" in 2007.
Estrone, along with total estrogen levels, is relatively low at the beginning of the menstrual cycle, then rise as the egg within the ovary matures. Merck indicates that levels are highest when the egg is fully mature, then drop again after the release of the egg from the ovary during ovulation. Changes to the normal fluctuation levels of estrone within the body can lead to menstrual irregularities; a disruption of normal hormone cycling during menstruation can cause menses to be excessively heavy or absent, depending on the severity and nature of the hormonal change.
Estradiol
Estradiol is another form of estrogen. In women, it is produced by the ovaries in the absence of pregnancy; in men, it's produced at lower levels within the testes. Estradiol production occurs during the fertile phase of a woman's life and plays a role in supporting fertility. When a woman enters menopause, estradiol production declines and overall levels drop significantly, according to the University of Chicago. Low estradiol levels can lead to the appearance of some of the side effects of menopause, including decreased vaginal lubrication and lowered sex drive. Abnormally high estradiol levels also have adverse effects; women producing excess estradiol are at an increased risk of developing breast cancer or heart problems.
Estriol
Estriol is a form of estrogen that is associated with pregnancy. Estriol is secreted by the placenta, which is the tissue within the womb that nourishes and supports the growth of the fetus. Some estriol is also generated by the fetus during embryonic development. The University of Washington indicates that estriol accounts for 90 percent of circulating estrogens in normal pregnancies, and that these levels can act as useful indicators for determining the health and well-being of the developing fetus and the functioning of the placenta. Low estriol levels may indicate developmental problems in the fetus or pregnancy failure, including miscarriage or stillbirth.
