Estrogens (also oestrogens) are a group of steroid compounds, named for their importance in the oestrus cycle, functioning 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 estrogens in women are estradiol, estriol and estrone. From menarche to menopause the primary estrogen is estradiol 17beta. 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.

Estrogen production

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Estrogen is 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 the liver, adrenal glands and the breasts. These secondary sources of estrogen 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 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 estrogen 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 estrogens (estradiol) and androgens such as testosterone and 5α-dihydrotestosterone. Estrogens can be produced by the enzyme aromatase which converts androgens such as DHEA to estrogens, mainly estradiol and estrone. Estriol is the third major human estrogen.

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