We will begin our survey of the major hormones that have been used in hormone replacement therapy with the sex steroid, estrogen. For one thing, most readers will have heard something of estrogen replacement therapy, and its astounding benefits. For another, estrogen, in particular, points up the need for caution in employing replacement therapy. Much of what we have learned concerning the need for balanced levels of all the hormones working together, has come about from early work with estrogen.

For example, it was found that to achieve success in eliminating problems involving the menstrual cycle - especially mood swings - it was necessary to achieve suitable levels of both estrogen and another hormone, progesterone. Also, since estrogen replacement alone has been linked to some cancers, especially uterine cancer, the use of estrogen here again points up the need for caution.

Estrogen is a female hormone, but levels occur in males as well. Its role in women is largely concerned with their development from girlhood onward, and in the maintenance of female traits and characteristics, as well as regulating their cycles of ovulation. Estrogen is produced mainly in the adrenal glands and the ovaries, and the levels of estrogen are under the control of the pituitary gland. It is derived from cholesterol via its precursor, pregnenolone, which is produced in the mitochondria of the cells.

In humans the main types of estrogen are estriol, estradiol, and estrone. These estrogens are each quite different and for this reason are used differently in therapeutics. For osteoporosis, estradiol has the greatest effect. However, this estrogen has the maximum effect on breast estrogen receptors. Estradiol has 1000 times the affinity for breast receptors over estriol, and for this reason it is avoided if possible in women with painful breasts, or with a high risk of cancer. Estrone is easily converted to Estradiol, so the dangers for estrone are the same.