Testosterone is usually reported as a single number: Total Testosterone. That number is the sum of every form the hormone takes in circulation, bound and unbound together.
Within that total sits the Active Fraction — the portion that is unbound and able to act on tissue. It is a separate quantity, and it is what this post is about.
The 98%: Locked vs. Unlocked
Your body is a master of transport and regulation. At any given moment, roughly 98% of the testosterone circulating in your bloodstream is "locked." It is tightly bound to two specific proteins:
- SHBG (Sex Hormone-Binding Globulin): Think of this as a high-security transport vehicle. Once T is bound here, it's largely unavailable for use.
- Albumin: A weaker bond, but still a barrier to immediate action.
Because these proteins are too large to pass through cell membranes, the testosterone they carry stays in the "garage." It's there, but it isn't working.
The 2% Rule: What is "Free" Testosterone?
The remaining 2% to 3% is what we call free testosterone.
Because it isn't tethered to transport proteins, this fraction is "biologically active." This is the only version of the hormone that can freely enter your cells and bind to androgen receptors, which is where processes like these begin:
- Muscle protein synthesis: Repairing and building tissue.
- Neurotransmission: Part of how focus and cognitive "snap" are regulated.
Why we use saliva testing
Saliva-draw testing carries the free fraction. Only the "free" (unbound) hormones are small enough to pass from the blood into the salivary glands, so a saliva sample acts as a natural filter: it shows what is bioavailable at the moment you take the sample — the fuel that is actually on the road and ready to drive your system. A blood draw measures the total amount of hormone in circulation — bound and free together. The two samples carry different fractions, and each is one input among many.
The Verdict
Free testosterone is the output. Reading your own result means knowing which fraction the number describes: the total in circulation, or the portion that is unbound and active.
If your biology is the engine, free testosterone is the fuel reaching it. That is the quantity a saliva measurement reports.
For research and wellness use only. Not for medical or diagnostic purposes.