Testosterone Free

Testosterone circulates in the blood 98% bound to protein. In men, 60% is bound with high affinity to sex hormone binding globulin (SHBG) and approximately 38% is bound weakly to albumin.The testosterone fraction that is bound to albumin dissociates freely in the capillary bed, becoming available for tissue uptake. Only 2 to 3% of testosterone exists in the unbound (free) state. All non-SHBG bound testosterone is considered to be bioavailable.

The SHBG-bound fraction is biologically inactive because of the high binding affinity of SHBG for testosterone. Free testosterone measures the free fraction, while bioavailable testosterone includes free plus weakly bound to albumin.

The reference methods for measurement of free testosterone involve equilibrium dialysis or ultrafiltration. These methods that are labor intensive, expensive, and available only in specialized laboratories. Although automated immunoassays for free testosterone are more widely available , they are inaccurate. The Endocrine Society recommends against their use . 

When an accurate immunoassay is used to measure total testosterone, calculation of free testosterone using a formula that incorporates sex hormone binding globulin (SHBG) correlates well with the result obtained by equilibrium dialysis.

The most Vermeulen formula is the most widely recommended and clinically validated formula for calculating free testosterone. The calculation is available through online tools such as the ISSAM calculator.  The calculator requires the input of total testosterone, SHBG, albumin concentrations. 

https://www.issam.ch/freetesuit.htm

The reference range of free testosterone has not been clearly established, but a lower limit of normal of 50 to 65 pg/mL has been suggested.

Serum total and free testosterone concentrations are typically concordantly low in hypogonadal men with a BMI less than 25. However, factors associated with low serum SHBG may result in discordant results in eugonadal men. In these clinical situations, total testosterone is low while free testosterone is normal. The most common causes of low serum SHBG are obesity, type 2 diabetes, and insulin resistance.

References

Anawalt BD et al. Adult Male Hypogonadism: A Review, JAMA 2026;335(24):2146-2159.

Bhasin S, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline, J Clin Endocrinol Metab, 2018;103(5):1715-1744.

Anawalt BD, Handelsman DJ, Debate about the free testosterone hypothesis: useful tool or misguided mirage? J Clin Endocrinol Metab. 2025;110(10):e3536-3540.

NarinxN, etal.Role of sex hormone-binding globulin in the free hormone hypothesis and the relevance of free testosterone in androgen physiology. Cell Mol Life Sci, 2022;79(11):543.

Jasuja R, et al, Reference intervals for free testosterone in adult men measured using a standardized equilibrium dialysis procedure. Andrology. 2023;11(1):125-133.