What it measures.
Free testosterone is the 2-4% of total testosterone that circulates unbound and can directly enter cells to activate androgen receptors. When SHBG (sex hormone binding globulin) is elevated, free testosterone may be low even with normal total testosterone, explaining symptoms despite 'normal' labs.
The concentration of testosterone not bound to proteins (SHBG or albumin). This fraction freely crosses cell membranes to exert biological effects.
Why it matters.
Free testosterone provides insight into actual hormone availability at the tissue level. Total testosterone can be misleading when SHBG is abnormally high or low. Free T better correlates with symptoms and clinical response.
Physiology.
About 44% of testosterone binds tightly to SHBG, 50% binds loosely to albumin, and only 2-4% circulates free. The albumin-bound and free fractions together constitute 'bioavailable testosterone.' SHBG increases with age, liver disease, thyroid disorders, and estrogen, reducing free testosterone.
Testing & preparation.
How to prepare
- Test between 7-9 AM
- Fasting preferred
- Equilibrium dialysis is gold standard method
- Calculated free T from total T and SHBG is acceptable alternative
When to test
When total testosterone is normal but symptoms suggest low T, or when SHBG may be abnormal (aging, obesity, liver disease, thyroid disorders).
How often
When total T doesn't explain clinical picture; annually if on TRT.
Interpretation.
High free testosterone
Common causes:
- Testosterone therapy
- Low SHBG (obesity, insulin resistance)
- Anabolic steroid use
Implications:
- May indicate supraphysiologic exposure
- Higher conversion to estradiol and DHT
- Monitor for side effects
Low free testosterone
Common causes:
- High SHBG (aging, liver disease, thyroid disorders)
- Primary or secondary hypogonadism
- Normal total T with elevated SHBG
Implications:
- Symptoms of low testosterone despite 'normal' total T
- Reduced tissue-level hormone action
- May benefit from therapy even with normal total T
Optimization.
Diet
- Address insulin resistance to lower SHBG if elevated
- Adequate protein and healthy fats
- Avoid excess alcohol
Lifestyle
- Resistance training
- Quality sleep
- Stress management
- Maintain healthy body weight
Supplements
- Boron may modestly reduce SHBG
- Stinging nettle root
- Address any nutrient deficiencies
FAQs.
Why is my free testosterone low when total testosterone is normal?
This common pattern usually indicates elevated SHBG, which binds more testosterone, leaving less free. SHBG increases with aging, liver disease, hyperthyroidism, low insulin, and certain medications. Even though total testosterone looks fine, less is actually available to your tissues, causing symptoms. Treatment may involve addressing the underlying cause of high SHBG or testosterone therapy.
How is free testosterone measured?
Two methods: (1) Direct measurement via equilibrium dialysis—the gold standard but expensive and not widely available. (2) Calculation from total testosterone, SHBG, and albumin levels using validated formulas. Calculated free T correlates well with dialysis measurement and is more practical for routine use.