Estrogen is a hormone produced by the follicles used to prepare the uterus for the reception of a fertilised egg. It is considered to be one of the primary female hormones just as testosterone is in males.
- estrogen receptors located in the hypothalamus serve as a master switch to control food intake, energy expenditure and body fat distribution 
- The woman's ovaries produce most estrogen hormones, although the adrenal glands also produce small amounts of the hormones.
There are several types of estrogen:
- Estradiol (main form of estrogen)
- Estriol (large amounts during pregnancy)
- Estrone (predominate in post-menopausal women)
Several artificial forms such as ethinyl-estradiol have been manufactured for use in oral contraceptive devices. These are often delivered via esters, such as menstrone.