What is Free Testosterone?

Free Testosterone is what tissues actually encounter, and it can move while Total Testosterone stays still. SHBG, sex hormone binding globulin, binds most of the hormone, so when the amount of that protein changes, the same total divides into a different free share.

Three figures go into the calculation: Total Testosterone, SHBG and albumin. FAI, the Free Androgen Index, estimates the same unbound share from Total Testosterone and SHBG alone, so albumin's loose binding is the only thing separating the two estimates.

Everything that moves Total Testosterone moves the free share with it, including the morning peak. Thyroid activity raises SHBG, while body weight and liver disease lower it, and more SHBG holds more of the hormone back, leaving a smaller free share.

Testosterone peaks in the early morning and falls through the day, so every estimate carries the hour it was taken, which is why blood is drawn in the morning and compared only with other morning samples. SHBG moves far more slowly, drifting across weeks and months. Yearly measurements are aimed at that drift, not at the morning rise and fall. This is Do Health's monitoring schedule, not a general recommendation for everyone.