Bound, free and total cortisol, and how to pick the right test.
You could have an extremely high cortisol on one test and a normal cortisol on another. That doesn't mean one test is accurate and the other isn't.
They may just be measuring different things.
Three kinds of cortisol
Most of the cortisol in your blood is bound: it travels attached to carrier proteins (CBG and albumin) and isn't active. A small fraction is free, and that's the cortisol your tissues actually respond to. Total cortisol is the two added together.
Why that matters
Anything that raises carrier proteins can raise your total cortisol without changing your free cortisol. Higher estrogen does exactly that, so pregnancy, birth control and estrogen therapy can all push a total-cortisol result up. A high total isn't the same as high cortisol exposure.
The five tests, in one line each
Serum total (blood): one moment, bound + free. Affected by estrogen.
Serum free (blood): one moment, active cortisol only.
Saliva: free cortisol, easy to repeat through the day, which shows your pattern.
24-hour urine: your total daily output, but not the ups and downs.
Hair: weeks to months of exposure, not your daily curve.
The takeaway
If you want to understand your cortisol pattern (when it rises and falls through the day), you need several measurements across one day. That's why I use saliva.
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This is education, not medical advice. Bring these questions to your provider: "Was this total or free cortisol? What time was it taken?"
— Sandra, The Holistic Health Engineer