What does the Testosterone, Free, Serum (Calculated or Direct) test measure?

Why is Testosterone, Free, Serum (Calculated or Direct) measured?

Free testosterone is the biologically actionable fraction and the most clinically meaningful testosterone parameter when SHBG alterations are present or suspected — the scenarios where total testosterone misrepresents the actual androgen exposure experienced by tissues. A male with total testosterone of 380 ng/dL and SHBG of 65 nmol/L has a free testosterone below the lower reference limit and will have symptomatic hypogonadism despite a technically 'low-normal' total testosterone. Conversely, a male with total testosterone of 380 ng/dL and SHBG of 15 nmol/L (from obesity and insulin resistance) has a free testosterone at the mid-reference range and no functional androgen deficiency. This is why the Endocrine Society's 2018 testosterone guideline recommends measuring free testosterone using a reliable method in all patients with conditions that alter SHBG — ageing, obesity, diabetes, liver disease, thyroid disease, HIV, OCP/HRT use. Free testosterone is the clinical standard for testosterone deficiency assessment in women because total testosterone assays are relatively insensitive at the low female concentration range; the free and bioavailable fractions provide the functionally relevant androgen index.

Reference ranges

Standard reference

Male (18–69): 46–224 pg/mL (1.60–7.77 ng/dL); Male (≥70): 6–73 pg/mL (0.21–2.53 ng/dL); Female (18–69): 0.2–5.0 pg/mL (0.01–0.17 ng/dL) # REVIEWER FLAG: Confirm LabCorp free testosterone reference ranges and methodology. The most important pre-publication clarification is whether LabCorp measures free testosterone by equilibrium dialysis (gold standard), direct analogue immunoassay (not endorsed by the Endocrine Society due to poor accuracy, particularly in females and elderly), or calculates it from total testosterone, SHBG, and albumin using the Vermeulen or similar equation (a validated indirect method). The methodology determines both the reference range and the clinical accuracy of the result. The Endocrine Society specifically recommends against direct analogue immunoassay for free testosterone. Confirm LabCorp's current methodology for this panel.

Functional medicine optimal

Male: 100–200 pg/mL (functional medicine optimal for metabolic, cognitive, libido, and musculoskeletal function support in working-age males); Female: 1.0–4.5 pg/mL (functional medicine optimal for libido, energy, and musculoskeletal maintenance in premenopausal women). # REVIEWER FLAG: Free testosterone optimal targets in functional medicine are not standardised — they vary by the assay method used. Reviewer to confirm framing and whether method-specific optimal thresholds should be cited.

Units

pg/mL, ng/dL, pmol/L

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.