The Physiology of Estrogen

Estrogen research has become the most exciting and rewarding of the issues surrounding women's health. Estrogen replacement, either alone (ERT) or in combination with progesterone (HRT), has an impact on many tissues in the body (Figure 8). Many of the symptoms or diseases associated with menopause and aging can be forestalled or improved with HRT/ERT. But until basic science research began to define the mechanism of estrogen action, estrogen was thought to have effects limited to the uterine and breast tissues. Now, however, there is growing scientific evidence that estrogen exerts beneficial actions in a number of tissues and systems including: bone and teeth, brain, eyes, vasomotor, heart, colon, and urogenital.(19-24)

Figure 7

In the early 1970s, estrogen was believed to be primarily a reproductive hormone with effects limited to the uterus and mammary glands. As the clinical utility of estrogen has become more commonplace, however, it is increasingly clear that it is a key hormone in the maintenance of skeletal integrity.

Figure 8

Other emerging benefits of estrogen include improved cardiovascular tone, cognition, cancer prevention, and protection against tooth loss and macular degeneration.

Historically, the concept that estrogen could have effects in so many organs was never taken seriously. Estrogen was thought to act primarily through a lock-and-key mechanism that involved a single estrogen receptor. But the following discoveries about estrogen action are changing this idea:

• There are at least 2 estrogen receptors (ER alpha and ER beta).
• Different estrogens appear to exhibit different activities in different cells.
• The potency and efficacy of a specific estrogen can vary from cell to cell and tissue to tissue.
• Estrogen receptors are more highly concentrated in some areas of the body.