What is free testosterone?
Educational only: This page is for general education—not personal medical advice, diagnosis, or treatment. See a licensed clinician for your situation.
Short answer
Free testosterone is the small fraction of testosterone in your blood that is not bound to proteins—mainly sex hormone-binding globulin (SHBG) or albumin. It is biologically active and available to tissues. Total testosterone includes bound plus free; you can have normal total testosterone but low free testosterone when SHBG is high.
Detailed answer
Most circulating testosterone is protein-bound. SHBG binds tightly; albumin binds loosely. Only free testosterone and the albumin-bound fraction are considered bioavailable.
Guidelines recommend measuring or calculating free testosterone when total testosterone is borderline or when SHBG may be altered—obesity, aging, thyroid disease, liver disease, or certain medications. Avoid relying on inaccurate direct free-testosterone immunoassays; equilibrium dialysis or validated calculations are preferred.
Symptoms such as low libido or fatigue require clinical context—sleep apnea, depression, and ADHD can mimic low testosterone even when labs look acceptable.
Evidence & references
- Endocrine Society hypogonadism guideline (2018)
- AUA testosterone deficiency guideline (reaffirmed 2024)
- Rosner et al., measuring testosterone position statement
Also read our Men's health articles · Dr. Sneh Pandey, MD