Plasma protein binding of norethisterone

Norethisterone (NET) is transported in the blood stream bound to plasma proteins. It is generally believed that only the part of the hormone that is not bound to plasma proteins can exert biological activity. NET binds to albumin and, like other 19-nortestosterone derived progestins, it also binds to sex hormone binding globulin (SHBG). The binding of NET to SHBG was studied in five women during a 21-day treatment cycle with an oral contraceptive containing 3 mg NET-acetate and 50 microgram ethinylestradiol. The total and SHBG-bound concentrations of NET increased in parallel with SHBG during treatment. The fraction not bound to SHBG and the free fraction of NET increased proportionately less, indicating that the total concentrations of NET measured by radioimmunoassay only in part reflect the biologically active fraction of the steroid.