When most people talk about testosterone, they focus on a single number: Total Testosterone. But in the world of endocrinology and performance, that number often hides the truth.
If you’ve ever felt the symptoms of low testosterone – fatigue, brain fog, or stalled progress in the gym – despite having "normal" bloodwork, the answer likely lies in the Active Fraction.
The 98% Problem: 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:
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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.
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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, bind to androgen receptors, and initiate the processes we associate with health and drive:
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Muscle Protein Synthesis: Repairing and building tissue.
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Neurotransmission: Regulating mood, focus, and cognitive "snap."
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Metabolic Health: Influencing how your body manages energy and fat distribution.
Salivawork: Why We Use Saliva Testing
Standard blood draws measure the total amount of hormone in the "garage." While useful, it doesn't always reflect what is actually available to your tissues at that exact moment.
Saliva-draw testing provides a unique window into your biology. Because only the "free" (unbound) hormones are small enough to pass from the blood into the salivary glands, a saliva sample acts as a natural filter. It shows us exactly what is bioavailable –the fuel that is actually on the road and ready to drive your system.
The Verdict
Free Testosterone is the output. To optimize your health, you have to look past the "Total" and understand what is actually active.
If your biology is the engine, Free Testosterone is the spark. Stop measuring the size of the tank, and start measuring the fuel reaching the engine.