Free Testosterone Index vs. Total Serum

The Vanity Metric

Most standard blood panels measure Total Testosterone. This is clinically misleading. Approximately 98% of circulating testosterone is bound to proteins: 60% to Sex Hormone Binding Globulin (SHBG) and 38% to Albumin. Testosterone bound to SHBG is biologically inactive—it cannot bind to androgen receptors.

Bioavailability: The "Free" Fraction

Free Testosterone represents the unbound fraction (~2%) that is actually available for tissue uptake and anabolic signaling. A patient can have high Total T but low Free T (due to high SHBG), resulting in hypogonadal symptoms despite "normal" lab results.

Albumin-Bound Testosterone

Testosterone bound to Albumin is held with low affinity (weak bond) and can dissociate in capillary beds to become bioavailable. Therefore, "Bioavailable Testosterone" is the sum of Free T + Albumin-bound T. This is the only metric that correlates with hypertrophy and libido.