Free Testosterone Calculator

Total testosterone can be misleading because it includes the protein-bound (inactive) fraction. Conditions affecting SHBG — obesity, aging, liver disease, medications — alter the ratio. A man with normal total T but high SHBG may have low free T and symptoms of hypogonadism, while someone with low SHBG may have normal free T despite borderline total T.

Sex Hormone-Binding Globulin (SHBG) is a protein produced by the liver that binds testosterone with high affinity. It effectively sequesters testosterone, making it unavailable for tissue uptake. SHBG increases with age, estrogen, liver disease, hyperthyroidism, and HIV, while it decreases with obesity, insulin resistance, hypothyroidism, and androgens.

The Vermeulen equation, published by Alex Vermeulen and colleagues in 1999, uses mass action principles to calculate free testosterone from total testosterone, SHBG, and albumin. It solves a quadratic equation derived from the equilibrium binding relationships. It correlates highly (r > 0.95) with equilibrium dialysis measurements.

Normal free testosterone for adult men is approximately 5-21 ng/dL (174-729 pmol/L), though ranges vary by laboratory and assay method. Levels decline with age: men in their 20s average 10-15 ng/dL, while men over 60 may average 5-9 ng/dL. Levels below 5 ng/dL generally indicate hypogonadism.

Direct measurement by equilibrium dialysis is the gold standard but is expensive, technically demanding, and available at few reference laboratories. Analog immunoassays for free T are inaccurate and not recommended by the Endocrine Society. Calculated free T using the Vermeulen method provides a reliable and accessible alternative.

Obesity decreases SHBG due to insulin resistance, which might suggest higher free T. However, obesity also independently decreases total testosterone production. The net effect is often low total T with a deceptively normal or slightly low free T. Aromatization of testosterone to estradiol in adipose tissue further compounds the problem.

Bioavailable testosterone is the sum of free testosterone and albumin-bound testosterone. Because albumin binds testosterone loosely (easily dissociable at the tissue level), this fraction is considered biologically available. It typically represents 50-60% of total testosterone and correlates well with clinical androgen status.

Yes, testosterone follows a circadian rhythm with peak levels in the early morning (6-8 AM) and a nadir in the evening. The variation can be 20-30%. This is why blood draws for testosterone should be done before 10 AM. The diurnal variation diminishes with aging.

Check free testosterone when: total testosterone is borderline (200-400 ng/dL in men), clinical suspicion of hypogonadism with normal total T, obesity or conditions known to alter SHBG, evaluating androgen excess in women (PCOS), monitoring hormone therapy, or when total T results are discordant with symptoms.