Estriol is a female sex hormone produced by the placenta during pregnancy and is the major estrogen form produced in the normal fetus. In non-pregnant females, estriol is derived almost exclusively from 17b-estradiol. Estriol exists in biological matrices in unconjugated (9%) and conjugated forms (91%). Conjugation of estriol with glucuronic or sulfuric acid occurs in the liver, which aids in steroid solubility, allowing rapid elimination via the kidneys.