Saliva levels of oestradiol and progesterone in relation to non-protein-bound concentrations in blood during late pregnancy

Serum and saliva samples were obtained from 25 women in the last eight weeks of pregnancy. The concentrations of oestradiol and progesterone were measured by radioimmunoassay. The proportion of each hormone which was not bound to protein in serum was measured by centrifugal ultrafiltration: for progesterone the unbound fraction was 2.5% (2.13--2.78%) and for oestradiol 1.27% (1--1.83%). There was only a weak relationship between the free hormone concentrations estimated in blood and the levels measured in saliva. We conclude that, for the situation examined here, saliva does not provide a useful measure of unbound, biologically active steroid.