Transcortin: A Corticosteroid-Binding Protein of Plasma. X. Cortisol and Progesterone Interplay and Unbound Levels of These Steroids in Pregnancy

The concentrations of total cortisol, progesterone and transcortin rise during pregnancy. Unbound cortisol shows a gradual increase during pregnancy, its level rising to about 3 times the concentration in normal plasma and in proportion to the rise of the total cortisol concentration. The level of unbound progesterone rises in the course of pregnancy, approximately in proportion to the total progesterone concentration. Progesterone replaces transcortin-bound cortisol increasingly during pregnancy until, in its later stage, the ratio of transcortin-bound progesterone to transcortinbound cortisol becomes about 0.3. The computed value for the association constant of the cortisol-transcortin complex in pregnancy plasma is about 12 μm−1 and that of the progesteronetranscortin complex is about twice this value.