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Oestrogens (also estrogens in American English) are a group of steroid compounds that function as the primary female sex hormone. While estrogens are present in both men and women, they are usually present at significantly higher levels in women of reproductive age. They promote the development of female secondary sex characteristics, such as breasts, and are also involved in the thickening of the endometrium and other aspects of regulating the menstrual cycle. Follicle stimulating hormone (FSH) and luteinizing hormone (LH) regulate the production of estrogen in ovulating women. Since estrogen circulating in the blood can feedback to reduce circulating levels of FSH and LH, some oral contraceptives contain estrogens.
The three major naturally occurring oestrogens in women are estradiol, estriol and estrone. In the body these are all produced from androgens through enzyme action. Estradiol is produced from testosterone and estrone from androstenedione. Estrone is weaker than estradiol, and in post-menopausal women more estrone is present than estradiol.
Oestrogens are produced primarily by developing follicles in the ovaries, the corpus luteum and the placenta. Some estrogens are also produced in smaller amounts by other tissues such as liver, adrenal glands and the breasts. These secondary sources of oestrogen are especially important in post-menopausal women. Synthesis of oestrogenes starts in theca interna cells in the ovary, by the synthesis of androstenedione from cholesterol. Androstenedione is a substance of moderate androgenic activity. This compound crosses the basal membrane into the surrounding granulosa cells, where it is converted to estrone or estradiol, either immediately or through testosterone. There is evidence that a testosterone supplement can support female sexual desire (Braunstein et al, 2005). Many of studies of the role of sex steroid hormones on sexual desire have been done in naturally post-menopausal women or women who have had their ovaries surgically removed. Such studies have found better correlation between sexual desire and androgen levels than for oestrogen levels (Warnock et al, 2005). A clinical study found that women aged 18 to 44 who reported low sexual desire tended to have low levels of dehydroepiandrosterone sulfate (Davis et al, 2005). Dehydroepiandrosterone is an abundant sex steroid in women and like other steroids is efficiently sulfated. Dehydroepiandrosterone (DHEA) is a precursor steroid that can be converted to oestrogens (estradiol) and androgens such as testosterone and 5α-dihydrotestosteroneTemplate:Ref.
A range of synthetic and natural substances have been identified that possess oestrogenic activity. These include bisphenol-A, phthalate esters and nonylphenol. The use of oestrogenic compounds, especially together with a progestagen, is a treatment for the symptoms of menopause. Among the older postmenopausal women studied as part of the Women’s Health Initiative (WHI), an orally-administered oestrogen supplement has been associated with an increased risk of dangerous blood clotting. The WHI studies used one type of oestrogen supplement, a high oral dose of conjugated equine estrogens (Premarin alone and with Provera as Prempro) . Research is underway to determine if risks of oestrogen supplement use are the same for all estrogen supplement types. In particular, topically-applied oestrogen may have a different spectrum of side-effects than does estrogen administered by the oral route. 
Oestrogen and lung disease
Among people over 70 who have never smoked, women make up 85 percent of those with chronic obstructive pulmonary disease (COPD). Mice studies suggest the possibility that COPD incidence may be tied to decreases in oestrogen as women age. (Female mice that had their ovaries removed to deprive them of oestrogen lost 45 percent of their working alveoli from their lungs. Upon receiving oestrogen, the mice recovered full lung function.)
Two proteins that are activated by oestrogen play distinct roles in breathing. One protein builds new alveoli, the other stimulates the alveoli to expel carbon dioxide. Loss of oestrogen hampered both functions in the test mice. (Massaro & Massaro, 2004)
- Nussey, S. and Whitehead, S., Metabolism of androgens to estradiol in Endocrinology: An Integrated Approach (2001) Published by BIOS Scientific Publishers Ltd. ISBN 1859962521
- Braunstein GD1, Sundwall DA, Katz M, Shifren JL, Buster JE, Simon JA, Bachman G, Aguirre OA, Lucas JD, Rodenberg C, Buch A, Watts NB., Safety and efficacy of a testosterone patch for the treatment of hypoactive sexual desire disorder in surgically menopausal women: a randomized, placebo-controlled trial. PMID 16043675
- Susan R. Davis, MD, PhD; Sonia L. Davison, MD; Susan Donath, MA; Robin J. Bell, MD, PhD, Circulating Androgen Levels and Self-reported Sexual Function in Women, Full text online (registration required).
- Massaro D1, Massaro GD., Estrogen regulates pulmonary alveolar formation, loss, and regeneration in mice, PMID 15298854
- Warnock JK1, Swanson SG, Borel RW, Zipfel LM, Brennan JJ; ESTRATEST Clinical Study Group., Combined esterified estrogens and methyltestosterone versus esterified estrogens alone in the treatment of loss of sexual interest in surgically menopausal women, PMID 16037752
- Oestrogen information from the US National Library of Medicine
- More Oestrogen information from Dimensions at the Castro-Mission Health Centre in San Francisco, California, US.