Bioavailable Testosterone

Bioavailable testosterone is the portion of your total testosterone that tissues can actually use — free testosterone plus the loosely albumin-bound fraction. It excludes the testosterone tightly bound to SHBG, which is effectively locked away and unavailable to cells. Because albumin binding is weak enough that the hormone can dissociate and act, bioavailable T is often cited as ~30–35% of total testosterone in a typical man.

Total testosterone is everything in the blood — SHBG-bound, albumin-bound, and free combined. Free testosterone is just the small unbound fraction, usually about 1–3% of total. Bioavailable testosterone sits in between: it's free T plus albumin-bound T, representing everything not locked to SHBG. So the hierarchy is total > bioavailable > free, and bioavailable is a useful middle-ground measure of what's biologically active.

It's calculated the same way as free testosterone — from total testosterone, SHBG, and albumin, using known binding affinities (the Vermeulen equation and similar formulas can output both). The math separates the tightly SHBG-bound portion from the rest; what remains is bioavailable. Because it depends on SHBG and albumin, two men with identical total T can have very different bioavailable levels.

A common adult male reference range is roughly 131–682 ng/dL, though ranges vary by lab and calculation method. As with free T, the number is only meaningful next to the assay or formula used, so compare against your own lab's range rather than across labs. Many TRT-focused clinicians find the upper third of the range tracks with a good symptomatic response.

They tell essentially the same story, since both exclude the SHBG-bound fraction and reflect what's usable by tissues. Free testosterone is more commonly ordered and discussed, so most men on TRT track that. If your panel happens to report bioavailable T, it's just as valid — use whichever your lab provides consistently and interpret trends over time with your physician.