I'm not sure if you need an explanation about the whole sentence or only about the "source term", but in any case here's the long version: liver normally causes the so called (in italian) "effetto di primo passaggio epatico", metabolizing the hormone and reducing the quantity of hormone available in the blood of the patient. If you bypass or at least reduce the metabolization of the hormone by the liver by administering the medicine in other ways (not orally but as a gel for example), you'll need lower doses of the hormone to reach normal oestradiol levels in the blood and to have a physiological E2/E1 ratio (E2 and E1 stand for two specific oestrogens that are often used in these pharmacologic preparations, oestradiol-17-beta and oestrone).