What exactly does a saliva hormone test measure: free or bio-available hormone?
Hormones pass into saliva from the cell membranes of the salivary gland. In other words, hormones must pass through saliva gland tissue to get into saliva, which means that a saliva hormone level measures delivery of hormone to tissue (cells) from the various reservoirs in blood. In contrast, a blood hormone level reflects hormone which has not yet been delivered to tissue. Because saliva reflects what actually gets into tissue rather than what might eventually get into tissue, it better reflects tissue (bio-available) hormone levels.
There is a common misconception that only free, unbound hormone is available to tissue. Research indicates that all forms of a given hormone are available to tissue to some extent. Free hormone, protein-bound hormone (albumin and SHBG-bound), red blood cell-associated hormone and conjugated hormone are all delivered to cells in varying amounts, depending on the type of tissue and whether the hormones are endogenous or exogenous. Free hormone levels in serum are known to underestimate bioavailable hormone levels, so measuring free hormones in serum may not accurately reflect what is being delivered to tissue.