What is bioavailable testosterone?

Bioavailable testosterone refers to the fraction of testosterone circulating in blood that cells can actually use. It consists of two components: free testosterone (roughly 1-3% of total) and testosterone weakly bound to albumin (the major carrier protein). Together, these represent hormone that is not tightly bound to sex hormone-binding globulin (SHBG) and can therefore cross cell membranes and interact with androgen receptors.

Total testosterone measures all forms of the hormone in circulation, including the large portion locked to SHBG, which cannot be accessed by tissues. Free testosterone alone captures only the unbound fraction. Bioavailable testosterone sits between these two measurements, offering a more complete picture of what your body actually has available for function.

This distinction matters in TRT because a man can have normal total testosterone while possessing low bioavailable levels if SHBG is elevated. Conversely, bioavailable testosterone can be insufficient despite acceptable free testosterone readings. Lab reports often calculate bioavailable testosterone from free testosterone and albumin values. TRT providers in Miami typically order bioavailable testosterone alongside free and total levels to make informed dosing decisions, since the bioavailable fraction better predicts clinical response and symptom relief.