Cholesterol is converted into pregnenolone by an enzyme called CYP11A1. Pregnenolone is the first building block for all steroid hormones. Pregnenolone converts to progesterone via two enzymes: - Three-beta -hydroxysteroid dehydrogenase) - delta-five-four-isomerase Progesterone can act as a hormone itself or continue down the pathway to convert to androstenedione, an androgen precursor, using enzymes: - 17αlpha-hydroxylase  - 17,20-lyase  Androstenedione can convert to testosterone via 17-beta -hydroxysteroid dehydrogenase or estrone via aromatase Estrone can then convert to estriol via 17-beta -hydroxysteroid dehydrogenase. Estradiol is the primary estrogen in reproductive-age women, while estrone is generally weaker and more prevalent after menopause. Or to estriol via 16αlpa-Hydroxyestrone which is a weaker estrogen than estradiol and estrone and is significant during pregnancy as it becomes the predominant estrogen produced by the placenta. Testosterone can convert to estradiol via aromatase or to to DDHT via 5 alpha reductase