Testosterone circulates in three forms: tightly bound to SHBG and unavailable, loosely bound to albumin, and free. Bioavailable testosterone is the free plus albumin-bound fractions together - the testosterone that can actually reach and act on your tissues. It is calculated from your total testosterone, SHBG and albumin (the Vermeulen method) rather than measured directly, and it reflects your usable testosterone more reliably than free testosterone alone, particularly when SHBG is high or low. That makes it a better marker than total testosterone when evaluating symptoms of low or high androgen activity.
Bioavailable testosterone
Usable testosterone, calculated from total testosterone, SHBG and albumin.