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

Catalog information reviewed: August 24, 2026 by the Sunridge Medical clinical team.

What this laboratory test evaluates

DetailsTestosterone, Free, Bioavailable and Total, Males (Adult), Immunoassay – 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.

What type of sample is required for this test?2 mL serum collected in a red-top tube (no gel)

Test ResourcesFor more information, please consult your healthcare provider.

Additional test information

Testosterone, Free, Bioavailable and Total, Males (Adult), Immunoassay – 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.

Collection and processing

• Sample type: Serum
• Fasting: No
• Estimated laboratory processing: Results should be available in 2 – 5 business days after the sample is collected

How Sunridge uses laboratory information

Sunridge considers the medical question, symptoms, history, medications, prior results, and whether the finding could change care before recommending testing. A laboratory result is not interpreted in isolation and does not establish a diagnosis by itself.

Before requesting this test

Availability, collection requirements, eligibility, and processing estimates can change. Sunridge reviews each request and, when appropriate, creates the order through Fullscript. Follow the current instructions supplied with the approved order.