What is bioavailable testosterone?

Bioavailable testosterone consists of testosterone that is biologically active and accessible to tissues: the free fraction (unbound) plus testosterone loosely bound to albumin. While most testosterone in the blood attaches to sex hormone-binding globulin (SHBG), only the free portion and the albumin-bound portion can readily cross cell membranes and produce physiological effects. Total testosterone alone can be misleading because SHBG levels vary by age, liver function, and other factors, meaning a patient with normal total testosterone may have low bioavailable testosterone if SHBG is elevated.

Bioavailable testosterone is calculated using total testosterone, SHBG concentration, and albumin levels through indirect equations or direct lab measurement, though direct assays are less common in routine clinical settings. The calculation accounts for the equilibrium between bound and free hormone fractions. For low testosterone diagnosis, bioavailable testosterone often provides a clearer picture than total testosterone alone, particularly in older men, those with liver disease, or patients taking medications that affect SHBG binding.

Practitioners use bioavailable testosterone to confirm symptomatic low T, distinguish true hypogonadism from SHBG-driven changes, and guide treatment decisions. When total testosterone is borderline or SHBG is abnormal, bioavailable testosterone helps avoid overtreatment or undertreatment. Many hormone therapy providers measure bioavailable testosterone as part of comprehensive hormone panels before starting testosterone replacement therapy.