What is Testosterone, Bioavailable (Albumin-Bound + Free), Calculated?
Bioavailable testosterone encompasses both the freely circulating unbound fraction and the albumin-loosely-bound fraction — the two fractions collectively available for uptake by target tissues. Albumin-bound testosterone, though protein-bound, dissociates rapidly from albumin's low-affinity single binding site as blood transits through tissues, making it effectively available for cellular uptake during capillary transit — the 'free hormone hypothesis' extended to weakly-bound fractions. SHBG-bound testosterone, by contrast, dissociates very slowly and is considered non-bioavailable at standard physiological conditions. Bioavailable testosterone therefore captures a larger and arguably more complete measure of androgen availability than free testosterone alone, particularly in conditions where total testosterone is normal but SHBG is substantially altered.