[Dynseli]

http://www.breastnexus.com/showthread.php?tid=21747 'Serum Estriol and Estetrol'
http://www.altmedrev.com/publications/3/2/101.pdf 'Estriol: Safety and Efficacy'

"Estriol has a much lower affinity for binding to SHBG; therefore, a greater percent is available for biological activity." [than estradiol]

"While estriol appears to be safer than estrone and estradiol and, in some situations, provides some protection against carcinogenesis, it appears its use in breast cancer patients with active disease or with patients at high risk for breast cancer should be approached with caution."

Pathways:
Estradiol or Estrone + 16a-OHase --> Estriol

16α-Hydroxyandrostenedione + 17-OH-SDH or aromatase --> Estriol

The combination of enzimes 16a-OHase, 17-OH-SDH, aromatase, and 3b-OH-SDH are important for the production of estriol.

Abbreviations:
3β-OH-SDH = 3b-Hydroxysteroid dehydrogenase
17-OH-SDH = 17-Hydroxysteroid dehydrogenase
16α-OHase = 16a-Hydroxylase
Estrone (E1)
Estradiol (E2)
Estriol or Oestriol (E3)
Oestetrol, Estetrol, or 15-alpha-hydroxyoestriol (E4)