Total vs Free vs Bioavailable Testosterone

Total testosterone is every testosterone molecule in a blood sample; free testosterone is the roughly 1–2% that circulates unbound; and bioavailable testosterone is that free fraction plus the larger share loosely held by albumin (together about half of the total). The three numbers describe the same hormone but answer different questions, which is why a lab report can show a perfectly “normal” total while the testosterone your tissues can actually use is low.

TL;DR — key takeaways

• Total = everything in the sample: free + albumin-bound + SHBG-bound. It is the cheapest, most common test.
• Free = the ~1–2% bound to nothing. This is the fraction most directly available to receptors.
• Bioavailable = free + albumin-bound (~35–55% of total), because the loose albumin bond releases testosterone readily at the tissue.
• SHBG is the swing factor. High SHBG locks up more testosterone, so two men with the same total can have very different free and bioavailable numbers.
• Free and bioavailable testosterone are usually calculated from total, SHBG and albumin — not measured directly — which is exactly what a free-testosterone calculator does.

The three fractions, and why they differ

Testosterone does not travel through the bloodstream alone. The large majority is bound to two carrier proteins. Sex hormone-binding globulin (SHBG) grips testosterone tightly and holds roughly 40–60% of it in men; this fraction is essentially locked away and not available to tissues moment to moment. Albumin, the most abundant plasma protein, binds another large share but only loosely, so that testosterone dissociates easily where it is needed. A small remainder — about 1–2% — is completely unbound, the classic “free” testosterone.

Total testosterone sums all three. Free testosterone is the unbound sliver alone. Bioavailable testosterone combines the free fraction with the albumin-bound fraction, on the reasoning that both are accessible to target cells while the SHBG-bound pool is not. That single distinction — whether you count the albumin-bound pool — is the entire difference between “free” and “bioavailable.”

Why does this matter clinically? Because total testosterone can mislead when SHBG is abnormal. Obesity, insulin resistance and aging tend to lower SHBG, while hyperthyroidism, liver disease and aging can raise it. When SHBG drifts, the total can look reassuring while the free and bioavailable fractions tell a different story. That is why guidelines such as the Endocrine Society's recommend assessing free or bioavailable testosterone in men whose total falls in a borderline range or who have conditions that alter SHBG.

Side-by-side: the three measurements

Percentages are typical adult-male figures and vary between individuals and assays; treat them as orientation, not fixed constants.

Units add another layer of confusion. Total testosterone is reported in ng/dL (common in the US) or nmol/L (common elsewhere), SHBG almost always in nmol/L, and albumin in g/dL or g/L. Before any free or bioavailable calculation can work, every input has to be in the units the equation expects — mixing ng/dL with nmol/L is one of the most frequent mistakes people make when they try to do the maths by hand, and it is the first thing a calculator standardises for you.

Visualising the split

A single total testosterone bar splits into three slices. The bioavailable portion is the free slice plus the albumin slice combined.

How this is calculated

Laboratories can measure free testosterone directly by equilibrium dialysis — the reference method — but it is slow and costly, so most reports instead calculate free and bioavailable testosterone from three measured numbers: total testosterone, SHBG and albumin. The most widely used equation is the Vermeulen formula, which models the binding chemistry and is what the on-site calculator applies. The arithmetic below uses only exact, checkable conversions and ratios; the full Vermeulen calculation needs the calculator, but these examples show the logic and the unit handling around it.

Frequently asked questions