Estrogen is a generic term for estrus-producing compounds; the female sex hormones, which include the following:
- Estrone (E1)
- Estradiol (E2)
- Estriol (E3).
Estrogen is produced by the ovaries and in smaller amounts by the adrenal cortex (The outer segment of the adrenal gland located on top of each kidney), testes (testicles) and fetoplacental unit (the fetus and the placenta).
Estrogen is considered to be the "female" hormone, whereas testosterone is considered the "male" hormone. However, both hormones are present in both sexes.1
Estrogen hormones play an essential role in the growth and development of female secondary sexual characteristics such as breasts, pubic and armpit hair, endometrium, regulation of the menstrual cycle and the reproductive system. During the menstrual cycle, estrogen acts to produce an environment suitable for fertilization, implantation, and nutrition of the early embryo.
In males, estrogen plays a role in normal male reproductive processes. Aspects of pubertal development in boys (growth of the long bones, their mineralization and epiphyseal closure) attributed to the actions of androgens are now recognized as being mediated in part by estrogen.2 The final steps of spermatid maturation, spermiogenesis, appear to be particularly sensitive and dependent upon estrogen.3
Uses for estrogen as a drug include:
- Birth control
- Hormone replacement therapy
- Advanced prostate or postmenopausal breast carcinoma treatment
- Osteoporosis prophylaxis.
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Fast facts on estrogen
Here are some key points about estrogen. More detail and supporting information is in the main article.
- Estrogen refers to a family of hormones, including estrone, estradiol and estriol which are primarily responsible for female reproductive processes and secondary sexual characteristics in mammals.
- Estrogen is produced by the ovaries and in smaller amounts by the adrenal cortex, testes and fetoplacental unit.
- Although estrogen is considered to be the "female" hormone it is present in both sexes.
- Estrogen hormones play an essential role in the growth and development of female secondary sexual characteristics.
- At puberty, a female's ovaries begin releasing estrogen hormone in synchronization with each monthly menstrual cycle.
- In males, estrogen assists in maturation of sperm and is important for normal male reproduction.
- Estrogen is likely involved in mood disturbances such as premenstrual syndrome (PMS).
- Synthetic estrogen is commonly used in birth control pills, contraceptives and hormone replacement therapy.
- HRT helps to restore female hormone levels to premenopausal levels, which can relieve some symptoms experienced after menopause.
- Estrogen is required during pregnancy for stimulating the production of progesterone from the placenta.
What is estrogen?
At puberty, a female's ovaries begin releasing estrogen hormone in synchronization with each monthly menstrual cycle.
Estrogen is produced by the ovaries and in smaller amounts by the adrenal cortex and fetoplacental unit.
The estrogen level rises suddenly mid-cycle, which triggers the release of an egg (ovulation). The estrogen level then quickly decreases after ovulation.
Hormones are chemical messengers that tell specific tissues to behave in a certain way. Estrogen is secreted by one tissue, travels by way of fluid, usually through the bloodstream, interacts with cells in a variety of target tissues in the body and delivers a message - in the case just mentioned, the message is "ovulate."
The three related hormones that make up the family known as estrogen include estrone, estradiol and estriol.
Estrone is considered a weaker form of estrogen and is the major estrogenic form found in naturally-menopausal women who are not taking HRT. It is the only estrogen that is present in any amount in women after menopause.
Estrone has a chemical name of 3-hydroxy-estra-1,3,5(10)-trien-17-one with the chemical formula C18H22O2.5 Estrone is the least abundant of the three hormones. Small amounts of estrone are made throughout the body in most tissues, especially fat and muscle.
Estrone is the major estrogenic form found in naturally-menopausal women who are not taking HRT. It is the only estrogen that is present in any amount in women after menopause.
Estradiol is the most potent form of estrogenic steroids produced by ovaries and exerts the fullest range of estrogenic effects. When estradiol reaches the tissues it connects with estrogen receptors to trigger specific activities in those tissues and cells.
Estradiol's chemical name is estra-1,3,5(10)-triene-3,17β-diol and it has the molecular formula of C18H24O2.
In addition to being produced by ovaries, estradiol can also be produced by conversion from a number of precursors in the adrenal glands and the placenta.
Estriol is a metabolic waste product of estradiol metabolism that can still have some effects upon a limited number of estrogen receptors.
Estriol has a chemical name of estra-1,3,5(10)-triene-3,16α,17β-triol and a molecular formula of C18H24O3.
Estriol is only produced in significant quantities during pregnancy.
Estriol is made by the placenta from 16-hydroxydehydroepiandrosterone sulfate (16-OH DHEAS)4, which is an androgen steroid made in the fetal liver and adrenal glands and is 8% as potent as estradiol and 14% as potent as estrone.
How does estrogen work?
Estrogens have an effect on target tissues by binding to fractions of cells called estrogen receptors. These receptors are protein molecules found inside those cells that are targets for estrogen action. Only estrogens (or closely related molecules) are able to bind to these receptors.
The target tissues affected by estrogen molecules all contain estrogen receptors; other organs and tissues in the body do not. Therefore, when estrogen molecules circulate in the bloodstream and move throughout the body, they exert effects only on cells that contain estrogen receptors.6
Estrogen receptors exist in the cell's nucleus, together with DNA molecules.
In the absence of estrogen molecules, these estrogen receptors are inactive and have no influence on DNA (which contains the cell's genes). When an estrogen molecule enters a cell and passes into the nucleus, the estrogen binds to its receptor, in doing so causing the shape of the receptor to change. This estrogen-receptor complex then binds to specific DNA sites, called estrogen response elements, located near genes that are controlled by estrogen.
After attachment to estrogen response elements in DNA, this estrogen-receptor complex binds to coactivator proteins, which then activates genes nearby. The active genes produce molecules of messenger ribonucleic acid (RNA), which guide the synthesis of specific proteins. These proteins can then influence cell behavior in different ways, depending on the cell type involved.6
Where does estrogen come from?
The ovaries are the prime location for estrogen production. The estrogens are released from the ovarian follicles and are also secreted by the corpus luteum (the temporary endocrine structure involved in ovulation and early pregnancy) following the release of an egg from the follicle and the placenta.
Follicle-stimulating hormone (FSH) causes estrogen release and in return estrogen inhibits FSH. The pituitary gland produces FSH in the brain.
Follicle-stimulating hormone (FSH) causes the release of estrogen and estrogen inhibits FSH as part of a negative feedback loop. The pituitary gland produces FSH in the brain.
Key functions of these hormones and their interactions with each other are listed below:
- Causes an egg to mature in an ovary
- Stimulates the ovaries to release estrogen.
- Encourages the growth of the uterine lining
- Inhibits FSH production to ensure that only one egg matures in a cycle
- Stimulates the pituitary gland to release the luteinising hormone (LH)
- Inhibits LH after ovulation.
- Stimulates egg release (ovulation)
- Stimulates estrogen and progesterone production.
Other sites of estrogen production in humans include:
- Adrenal cortex
- Fat cells
On the next page we look at what estrogen does, the menstrual cycle and the roles of synthetic estrogen in medicine. On the final page we look at the role of estrogen during pregnancy and menopause. We also discuss estrogen in men and estrogen's implicated role in the development or progression of diseases.