Free testosterone is the unbound, biologically active sliver of total testosterone, about two percent of the whole. It cuts through the SHBG distortion that makes total readings misleading in obesity, aging and thyroid disease.
Standard range versus optimal range
| Framework | Free testosterone value | Source |
|---|---|---|
| Adult men, typical reference | about 5-21 pg/mL, assay dependent | Endocrine Society references |
| When to measure or calculate | whenever total testosterone conflicts with the picture | Endocrine Society 2018 |
| Preferred method | calculated from total T, SHBG and albumin, or equilibrium dialysis | Endocrine Society 2018 |
When it runs high
High free testosterone in women produces androgen effects even when the total looks normal, because low SHBG unbinds more hormone. Acne, scalp hair thinning and excess facial hair with a normal total T is the classic free-testosterone story.
- Polycystic ovary syndrome with low SHBG
- Insulin resistance lowering SHBG
- Androgen supplementation
- Rare androgen-producing tumors
When it runs low
Low free testosterone means tissues are seeing little active hormone regardless of the total. This is the pattern that explains symptoms in obese men whose totals looked borderline, since their low SHBG had been masking the deficit in reverse.
- True hypogonadism of testicular or pituitary origin
- High SHBG states binding up the hormone, from aging, thyroid excess or liver disease
- Severe caloric restriction and overtraining
- Opioid and glucocorticoid use
The two percent that does the work
Testosterone circulates in three pools, tightly bound to SHBG, loosely bound to albumin, and a free fraction of one to three percent that enters cells and activates the receptor. Physiologists consider the free plus albumin-bound share as the bioavailable portion. When SHBG is normal the total tracks the free well enough, but when SHBG swings, the total lies. A 60-year-old with high SHBG can show a comfortable total while tissues starve, and an obese 30-year-old shows the mirror image.
Calculated beats measured for most labs
Direct analog free testosterone assays, the cheap ones on many lab menus, are notoriously inaccurate and guidelines advise against them. The trusted routes are equilibrium dialysis, the reference method, and the Vermeulen calculation from total testosterone, SHBG and albumin, which agrees well with dialysis in validation studies. Our free testosterone style calculators implement exactly this math.
Putting it to work
Order or calculate free T whenever the total sits near a decision boundary, whenever SHBG-shifting conditions are present, and whenever symptoms and total disagree. Interventions follow the underlying driver, weight loss raises free T by restoring SHBG and production together, overtraining and sleep debt lower it, and confirmed deficiency with symptoms is where replacement discussions begin.
Related calculators
Related biomarkers Total testosterone · SHBG · LH