Produced primarily in the ovaries, corpus luteum, placenta and, to a lesser extent, the liver and the heart, estrogen is an essential hormone in the gonadal development and function of women, especially those in the reproductive age group. Estrogen is also found in males, though at a significantly lesser concentration than in females. It has three primary physiologically active forms namely estrone (E1), 17 Beta estradiol (E2), and estriol (E3). Of these three active forms, estradiol is the most potent as well as predominant in women who are post -pubertal and premenopausal.
The effects of estradiol in the body are mediated by alpha and beta estrogen receptors. The distribution of these alpha and beta estrogen receptors on various organs in the body dictate the varied actions of estradiol. Some of the many roles of estradiol in the human body include:
- Development of breast tissue during puberty.
- Development of the mammary ducts during pregnancy.
- Stimulation of lactation in breastfeeding mothers.
- Fusion of epiphyseal growth plates at puberty as well as the inactivation of osteoclastic activity, thereby minimizing the risks of developing osteoporosis.
- Maintenance of epithelial cells in the vulva and vagina in women of reproductive age.
- Reduction in blood levels of low-density lipoproteins (LDL) and serum cholesterol as well as an increase in high-density lipoproteins (HDL) and triglyceride levels. This reduces the risk of developing coronary artery disease.
- A negative feedback effect resulting in the reduction in blood levels of follicular stimulating hormone (FSH) during at the onset of female menstruation.
Estradiol has several routes of administration depending on clinician and patient preferences, and the clinical indication. Common routes of estradiol administration are oral, topical (transdermal), intra-muscular (as depot preparations), and vaginal. Depending on its route of administration, estradiol has a variable hepatic first pass effect which plays a significant role in its bioavailability.