Free testosterone
Medically reviewed by Hormone Journal Editorial Team · Last reviewed 2026-05-17
Free testosterone is the small fraction of testosterone in the blood not bound to proteins, making it immediately available to act on cells and tissues.
What it is
Free testosterone is the small fraction of testosterone in the blood not bound to proteins, making it immediately available to act on cells and tissues. Most testosterone in circulation — roughly 98% — is attached to carrier proteins: about 44% to sex hormone-binding globulin (SHBG, a protein that binds tightly) and around 54% to albumin (which binds loosely). Only 1–3% circulates as "free" testosterone. Because free testosterone can pass directly into cells and activate androgen receptors (proteins inside cells that respond to testosterone), it is often considered the biologically active fraction, though albumin-bound testosterone is also partially available and is sometimes grouped with free testosterone under the term "bioavailable testosterone."
Causes and mechanism
The pituitary gland signals the testes (in men) and ovaries and adrenal glands (in women) to produce testosterone. How much of that testosterone remains free depends heavily on SHBG levels. When SHBG rises — due to aging, liver disease, hyperthyroidism, or certain medications — more testosterone is bound and free levels fall, even if total testosterone looks normal. When SHBG falls — due to obesity, insulin resistance, hypothyroidism, or anabolic steroid use — free testosterone rises. Body fat content is independently and negatively correlated with free testosterone: higher adiposity increases aromatase enzyme activity, converting testosterone to estrogen and raising SHBG. Conversely, higher lean muscle mass and physical activity are positively associated with free testosterone levels.
Symptoms and diagnosis
Low free testosterone in men can cause reduced libido, fatigue, loss of muscle mass, increased body fat, mood changes, and impaired physical performance. Research from the Framingham Offspring Study found that low free testosterone was independently associated with mobility limitation and poorer physical performance in community-dwelling men. In women, elevated free testosterone is more clinically relevant: it is a key marker in polycystic ovary syndrome (PCOS) and has been shown to independently predict metabolic syndrome severity in adolescent girls aged 12–19. Symptoms of excess free testosterone in women include irregular periods, acne, excess hair growth (hirsutism), and scalp hair thinning.
Diagnosis requires a morning blood draw (testosterone peaks between 7–10 a.m.). Free testosterone is measured either by equilibrium dialysis (the reference standard method) or calculated from total testosterone and SHBG using validated equations. Direct immunoassay kits for free testosterone are widely available but considered less accurate by the Endocrine Society.
Treatment options
Treatment targets the underlying cause rather than free testosterone in isolation.
- Low free testosterone in men: Testosterone replacement therapy (TRT) — available as gels, injections, patches, or pellets — is indicated when symptoms align with confirmed biochemical deficiency. Lifestyle changes (resistance training, weight loss) can raise free testosterone by reducing SHBG and body fat.
- Elevated free testosterone in women (e.g., PCOS): Combined oral contraceptives raise SHBG, lowering free testosterone. Anti-androgens such as spironolactone or flutamide reduce androgen action at the receptor level. Metformin and weight loss address the underlying insulin resistance that suppresses SHBG.
Researchers debate the threshold at which free testosterone becomes clinically low in older men, and no universally agreed cutoff exists across major guidelines.
When to see a clinician
Request a free testosterone test if you have symptoms of androgen deficiency or excess that persist beyond a few weeks, or if total testosterone results are borderline and SHBG is known to be abnormal. Women with irregular cycles, hirsutism, or signs of metabolic syndrome should ask about free and total testosterone as part of a PCOS workup. Men over 40 experiencing unexplained fatigue, significant muscle loss, or sexual dysfunction warrant evaluation. Always test in the morning and, where possible, on two separate days before acting on results.
FAQs
What is the difference between free testosterone and total testosterone?
Total testosterone measures all testosterone in the blood — bound and unbound. Free testosterone measures only the ~1–3% not attached to carrier proteins like SHBG or albumin. A person can have a normal total testosterone but low free testosterone if their SHBG is elevated, which is why clinicians often order both tests together.
What is a normal free testosterone level?
Reference ranges vary by lab and measurement method. For adult men, free testosterone typically falls between 5 and 21 ng/dL (or roughly 174–729 pmol/L) depending on age; levels decline by approximately 1–2% per year after age 30. For adult women, normal free testosterone is much lower, generally 0.3–1.9 ng/dL. Always interpret results against the specific lab's reference range and in the context of symptoms.
Can lifestyle changes raise free testosterone?
Yes. Research shows free testosterone is negatively correlated with body fat percentage and positively correlated with lean mass and physical activity. Resistance training, weight loss, and reducing alcohol intake can meaningfully increase free testosterone by lowering SHBG and reducing aromatase activity. However, lifestyle changes alone are unlikely to fully correct free testosterone in cases of primary hypogonadism (where the testes or ovaries themselves are not producing enough testosterone).
Is free testosterone testing covered by insurance?
Coverage varies by insurer and country. In the U.S., free testosterone testing is generally covered when ordered for a documented clinical indication such as suspected hypogonadism or PCOS, but prior authorization may be required. Equilibrium dialysis — the most accurate method — is more expensive and less commonly covered than calculated free testosterone. Check with your insurer before testing, as out-of-pocket costs can range from $30 to over $150 depending on the method.
Why might free testosterone be high even when total testosterone is normal?
Free testosterone rises when SHBG is low, because less testosterone is being bound. Conditions that suppress SHBG include obesity, type 2 diabetes, hypothyroidism, and use of anabolic steroids or glucocorticoids. In women, elevated free testosterone with normal total testosterone is a common finding in PCOS and insulin-resistant states, and it carries independent metabolic risk even when total testosterone appears unremarkable.
Sources
- Effects of Lifestyle, Diet, and Body Composition on Free Testosterone and Cortisol Levels in Young Men
- Free Testosterone Levels Are Associated with Mobility Limitation and Physical Performance in Community-Dwelling Men: The Framingham Offspring Study
- Free testosterone independently predicts metabolic syndrome severity in U.S. adolescent girls aged 12–19 years
- Endocrine Society Clinical Practice Guideline: Testosterone Therapy in Men with Hypogonadism
- Mayo Clinic: Testosterone — What It Does and Doesn't Do