Hormones · Bioavailable fraction

Free Testosterone(FT)

Free testosterone is the unbound, biologically active fraction of total T, the portion that crosses cell membranes and activates androgen receptors. Most circulating testosterone is bound to SHBG and albumin and is not immediately available to tissues. The free fraction is typically 1 to 3 percent of total T, but that small slice does most of the work.

Optimal range

150–250pg/mL

Clinical "normal"

50–210 pg/mL

Avg. cost (US)

$95

Test frequency

With total T, every 3–6 months

When to measure

Same draw as total T, fasted, 7–9 AM. Calculated values (from total T + SHBG + albumin) are usually preferred over direct assays.

How to measure

Calculated from total T + SHBG (Vermeulen formula) is gold standard. Direct assays exist but vary in accuracy.

Average cost

≈ $95 cash price. Often covered by insurance with relevant ICD-10.

Why this biomarker matters

Symptoms of androgen deficiency, low libido, erectile dysfunction, fatigue, reduced muscle mass, depressed mood, correlate more reliably with free testosterone than with total testosterone in most published cohorts. A man with a total testosterone of 600 ng/dL and a high SHBG of 80 nmol/L may have a calculated free T below 8 ng/dL and be symptomatic, while another man with a total of 450 ng/dL and a low SHBG of 20 nmol/L can have a free T of 12 ng/dL and feel asymptomatic. The Vermeulen calculation (free T derived from total T, SHBG, and albumin) is considered the practical gold standard for most clinical purposes; it correlates closely with equilibrium dialysis, the research reference method, at a fraction of the cost. Direct free T immunoassays are widely available but vary substantially between labs and are not recommended by the Endocrine Society for clinical decision-making. Typical reference ranges run from roughly 9 to 30 ng/dL in healthy adult men, with a steady age-related decline of roughly 1 to 2 percent per year after age 30. Optimal targets on TRT vary by clinician but generally aim for the upper half of the age-matched reference range rather than supraphysiologic levels.