Estrogen is the primary hormone that controls female reproductive function and secondary sexual characteristics in humans. Estrogen's main function in women is to control the menstrual cycle and pregnancy. As part of regulating this cycle, the major source of estrogen in pre-menopausal women is the ovaries. Some estrogen is also produced in fat tissue, as well as other sites in the body. These other areas of estrogen production are the major source in post-menopausal women and may also play a functional role in men.
Ovaries
All estrogen in humans is synthesized from androgen hormones, specifically the male hormone testosterone or a closely related male hormone called androstenedione. One type of cell in the ovaries creates the androgens, and another type of cell converts them into estrogen. This coordinated production of estrogen from different cell types in the ovaries is closely controlled by centers in the brain that regulate the female menstrual cycle. Extremely high levels of estrogen usually result from either tumors in the ovaries or in the control centers in the brain. After menopause, the ovaries become "exhausted" and no longer produce estrogen in response to the brain's signals.
Fat Tissue
Other parts of the body are also capable of converting androgens to estrogen. Fat tissue is the most significant site of androgen conversion other than the ovaries. In women, the adrenal glands normally produce small amounts of androgens, which are converted by fat tissue into estrogen. In women who are obese or who have a condition called polycystic ovary syndrome, in which the ovaries produce large amounts of androgens without converting them to estrogen, fat tissue can become a significant source of estrogen, disrupting the normal menstrual cycle and increasing the risk of endometriosis and endometrial cancer. Fat tissue can also produce significant amounts of estrogen in men who are very obese or who have very high levels of androgens due to tumors in the testes or due to the illicit use of androgenic anabolic steroids for increasing muscle growth. The estrogen produced under these circumstances can lead to breast enlargement in men.
Other Sites of Androgen Conversion
While only fat tissue and the ovaries produce enough estrogen to affect the body as a whole, other sites do convert sufficient androgens to have a local effect. For example, the bones of both men and women produce small amounts of estrogen, which may play a role in preventing bone loss and osteoporosis. Since men have much higher circulating levels of androgens than women, this local conversion to estrogen may explain why older men are less prone to osteoporosis than post-menopausal women, even though both normally have very low levels of estrogen in the blood. Breast tissue also converts small amounts of androgens to estrogen, which may play a role in post-menopausal breast cancer. Certain types of breast cancer tumors grow in response to estrogen stimulation, and the local production of estrogen in breast tissue may be the most significant source of estrogen encouraging the growth of these tumors in post-menopausal women.
References
- "Medical Physiology"; WF Boron and EL Boulpaep, eds.; 2003
- "Harrison's Principles of Internal Medicine" 16th edition; Dennis L. Kasper, et al., eds.; 2005
- Endocrine-Related Cancer: Local Estrogen Biosynthesis in Males and Females
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