Sulfates are the most important circulating conjugates of ethinyl estradiol and glucuronides predominate in urine. The main oxidative metabolite is 2-hydroxy ethinyl estradiol, formed by the CYP3A4 isoform of cytochrome P450. Part of the initial-pass metabolism of ethinyl estradiol is believed to come about in gastrointestinal mucosa. Ethinyl estra