A hormone, occurring in both human sexes and affecting mood and sexual characteristics, best mainly known as the most important female sex hormone. It is named from its ability to produce oestrus in animals, and is also spelt estrogen.

There are three naturally occurring forms of oestrogen in women:

When oestrogen is at its highest level in the blood, follicles release eggs. As well as regulating primary and sexual characteristics it regulates secondary ones such as the distribution of body fat. Falling levels of oestrogen may be linked with mood alternations such as PMT, post-natal depression, and menopausal depression.

There are also environmental oestrogens, including phytoestrogens such as those in tofu and broccoli, synthetic oestrogens such as ethinyl oestradiol (EE), prescribed oestrogens such as the contraceptive pill, residues in drinking water, and pesticides. Some of these may be toxic in the sense that they are associated with higher levels of cancer.

A hysterectomy removing the ovaries means oestrogen is not produced in sufficient quantities, so in this case the hormone is the one used in HRT. It may be used alone or with progestin.

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