Testosterone, Free, Bioavailable and Total, Males (Adult), Immunoassay details

This test panel may be helpful in assessing androgen status and screening for hypogonadism in men. This immunoassay cannot accurately measure the low testosterone levels generally found in women, children, and men with hypogonadism [1].

Testosterone circulates in 3 major forms: unbound (free), weakly bound to albumin, and tightly bound to sex hormone-binding globulin (SHBG). Free testosterone comprises 2% to 4% of total testosterone and is biologically active. Albumin-bound testosterone can dissociate from albumin in capillaries and become available to tissue. Thus, bioavailable testosterone refers to free testosterone plus albumin-bound testosterone. In this panel, free testosterone and bioavailable testosterone concentrations are calculated based on measurements of total T, SHBG, and albumin.

In men, fasting total testosterone concentrations measured in the morning are recommended for screening for hypogonadism [2]. In patients with conditions affecting SHBG concentrations or when total testosterone levels are near the lower limit, free testosterone levels may be more sensitive than total testosterone levels for assessing androgen status. When the total testosterone level is low, or the total testosterone level is normal but free testosterone is low, the fasting total testosterone should be confirmed on another morning [2]. This test may also be used to monitor testosterone replacement therapy.

Direct immunoassays cannot accurately quantify low concentrations of testosterone, especially concentrations under the lower limit of the normal range for men [1]. Chromatography/mass spectrometry is preferred for quantifying testosterone levels in children, women, or men with low testosterone levels [1]. Further studies are needed to elaborate the use of bioavailable testosterone concentrations.

Note: Dietary supplements containing biotin may interfere with this assay. Please refer to the collection instructions for more information.

The results of this test should be interpreted in the context of pertinent clinical and family history and physical examination findings.

References

1. Rosner W, et al. J Clin Endocrinol Metab. 2007;92(2):405-413.
2. Bhasin S, et al. J Clin Endocrinol Metab. 2018;103(5):1715-1744.