When natural progesterone or any of the sex hormones are rubbed on the skin, within two hours it is seen in the saliva. It obviously gets there through the blood and here is where it gets complicated. Because of their fat solubility, the hormones are carried in the lipophillic (fat loving) red cell mass that can host the fatty substance much better than the watery serum. Ordinary blood tests, however, only look in the watery portion of the blood and not the red cell mass and, therefore, do not register appreciable levels of the hormone. Then, too, the hormones that circulate in the blood are packaged in a protein wrap, and protein-bound hormones are biologically inactive whereas the hormone in saliva is biologically active because it’s not protein bound.
Therefore, the saliva hormones reflect only the biologically available hormones. Tests have shown that you can track the hormone levels in saliva as they rise starting in two hours up to their peak at about 18 hours and then drop. You can actually do your own testing with saliva kits.