Bioavailable testosterone refers to the fraction of total testosterone in the blood that is not tightly bound to sex hormone-binding globulin (SHBG) and is therefore available to enter cells and exert biological effects. It includes both free testosterone (completely unbound) and testosterone loosely bound to albumin, which can readily dissociate and enter tissues.

Approximately 98% of total testosterone circulates bound to proteins — about 44% to albumin and 54% to SHBG. SHBG-bound testosterone is tightly bound and cannot enter cells. Only the remaining 2% (free) plus albumin-bound testosterone is biologically active. Because SHBG levels vary significantly with age, obesity, liver disease, and medications, total testosterone can be misleading — bioavailable testosterone corrects for this and reflects what the body's tissues can actually use.