Progesterone in saliva does not parallel unbound progesterone in plasma

To assess the potential application of salivary progesterone, in light of the voluminous published information on blood progesterone, I compared plasma and salivary progesterone in paired samples and found that they exhibit similar patterns, both having high concentrations in the second half of the cycle. However, the ratio between values for plasma and saliva changed during the cycle. I observed no concomitant changes in binding of progesterone to plasma proteins, which might have been expected if the concentrations in saliva reflect those of non-protein-bound steroid in plasma. There was some binding of progesterone by salivary proteins, but the proportion of progesterone so bound apparently remained constant during the cycle. Data on progesterone in saliva must therefore be interpreted with awareness of the properties of saliva, and not simply as "free" plasma progesterone.