Estriol
Estriol is one of the three major naturally occurring estrogens, the others being estradiol and estrone. Estriol is produced almost exclusively during pregnancy and is the major estrogen produced in the normal human fetus. During pregnancy the production of estriol depends on an intact maternal-placental-fetal unit. Fetal-placental production of estriol leads to a progressive rise in maternal circulating levels reaching a late-gestational peak several orders of magnitude greater than non-pregnant levels. Estriol has been suggested to be less carcinogenic than estradiol and estrone in animal studies. It has been shown that at doses effective for the relief of postmenopausal symptoms, estriol does not induce endometrial proliferation to the extent of the other estrogens. Topical estriol has also been used for the relief of postmenopausal genital atrophic changes and urinary incontinence.

Interpretation of Results
High levels of estriol are seen in gestation. Otherwise the production of estriol is uniformly low regardless of menopausal status or gender. Estriol is measured in pg/ml

References
1) Buster JE: Gestational changes in steroid hormone biosynthesis, secretion, metabolism, and action.  Clin Perinatol 1983;10:527-552
2) Canez MS, Lee KH, Olive DL: Progestogens and estrogens. Infertil Reproduct Med Clin North Amer 1992;3:59-78
3) Yang TS, Tsan SH, Chang SP, Ng Ht: Efficacy and safety of estriol replacement therapy for climacteric women. Am J Obstet Gynecol 1995;173:670-671
4) Lemon HM, Kumar PF, Peterson C, Rodriquez-Sierra JF, Abbo KM: Inhibition of radiogenic mammary carcinoma in rats by estriol or tamoxifen. Cancer 1989;63:1685-1692
5) McGregor JA, Jackson GM, Lachelin GGL et al.: Salivary Estriol as Risk Assessment for Preterm Labor: A Prospective Trial. Am J Obstet Gynecol 1995;173(4);1337-1342