Let’s break down the physiology simply.
Saliva is an ultrafiltrate of blood plasma.
Only the unbound, free (bioavailable) hormone fraction can pass through salivary gland cells into saliva. Bound hormones remain confined to the bloodstream.
When we measure total blood levels, we capture both bound and unbound fractions.
But only the free fraction is physiologically active: it crosses cell membranes, binds receptors, and produces biological effects.
The same barrier that allows free hormone into saliva mirrors the process for entry into target tissues: brain, breast, uterine lining, muscle, and more.
Saliva therefore reflects the hormone fraction that has successfully reached the cellular compartment—where actual function occurs.
This is not about one test being superior. It is about compartment specificity:
Blood reflects transport and total circulating hormone.
Urine reflects metabolic clearance and elimination.
Saliva reflects bioavailable tissue exposure.
Every laboratory method measures a different physiological compartment. The critical question is:
Which compartment provides the most relevant answer to the clinical or functional question being asked?
Accurate hormone assessment begins with matching the test to the tissue-level outcome you need to understand.
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