Understanding free testosterone
Free testosterone refers to the portion of testosterone that is not bound to proteins in the blood. Most testosterone circulates attached to carrier proteins, and only a small unbound fraction is described as "free." This page explains, in plain language, what the free testosterone measurement represents, how it relates to total testosterone and the proteins that carry it, why free testosterone is method-dependent, and public reference examples from published sources.
Everything here is educational context. It does not interpret what any particular value means, does not diagnose, and does not describe any course of action.
What free testosterone is
Testosterone travels through the bloodstream in several forms. The total testosterone measurement reports the combined amount, while free testosterone reports only the unbound fraction.
How testosterone is carried in the blood
- SHBG-bound testosterone — testosterone attached tightly to sex hormone-binding globulin (SHBG). This portion is generally considered unavailable to tissues.
- Albumin-bound testosterone — testosterone attached more loosely to albumin. It is bound, but the connection is weaker than with SHBG.
- Free (unbound) testosterone — the small fraction not attached to any carrier protein. This is what the free testosterone measurement reports.
Because the amount of SHBG present varies between individuals, two people with a similar total testosterone measurement can have a different unbound fraction. This is why free testosterone is commonly discussed alongside total testosterone and SHBG rather than on its own.
Why it is commonly measured
Free testosterone is commonly measured as part of a broader look at androgen status on a hormone panel, usually together with total testosterone and SHBG. In research and clinical education it is reviewed to describe the unbound fraction rather than the total amount:
- Total vs free context — total testosterone reports the combined amount; free testosterone isolates the unbound portion.
- SHBG relationship — because SHBG binds testosterone, the amount of SHBG present shapes how much testosterone is bound versus unbound.
- Age and sex context — free testosterone differs by sex and can vary with age, so it is read within the context of the individual.
This page does not interpret what any particular value means and does not recommend any action. Any individual measurement should be discussed with a licensed healthcare provider.
Free testosterone is method-dependent
Unlike total testosterone, free testosterone is method-dependent: the figure you receive depends heavily on how it was measured. Common approaches include equilibrium dialysis (often described as a reference method), calculated free testosterone (estimated from total testosterone, SHBG, and albumin), and direct immunoassay.
Because these methods can produce different figures for the same sample, a free testosterone value is only meaningful next to the method and reference interval used by the laboratory that produced it. Always read free testosterone together with the method noted on the report.
Common units
Free testosterone is reported in more than one unit:
- pg/mL (picograms per milliliter) — commonly used for free testosterone.
- ng/dL (nanograms per deciliter) — also used by some laboratories.
- pmol/L (picomoles per liter) — an SI unit common in many other countries.
Units and assay methods vary by laboratory, so always read the unit and method printed next to your own measurement. A figure in one unit is not directly comparable to a figure in another unit without converting between them.
Public reference examples
The examples below are public reference examples drawn from published sources and labeled by the study population they came from. Because free testosterone is method-dependent, these figures reflect the specific method used in each source. No single range applies to everyone, and they are shown for educational context only.
Reference ranges vary by laboratory, assay, method, age, sex, and population. Free testosterone in particular is method-dependent, so figures from different methods are not directly comparable. These are public reference examples for education, not a universal standard and not a determination about any individual. Where a measurement is available, Pepvela prioritizes the reference range printed on your own lab report.
Age and sex context
Free testosterone reference intervals differ substantially between males and females, and they may also vary with age, the laboratory method used, and the population studied. The published male distribution above declines across older age groups, and the female example sits in a different range entirely.
For this reason, free testosterone is read within the reference interval that matches the individual, the assay method, and the specific laboratory performing the measurement, rather than against a single fixed number.
Related biomarkers
These related biomarkers are often reviewed alongside free testosterone for educational context:
Free testosterone and total testosterone describe different fractions of the same hormone and are usually read together with SHBG.
Related peptide research
Research literature involving certain compounds may measure testosterone-related markers as endpoints. The following research pages are provided for educational context only and make no claim about any effect on testosterone:
Citations
- MedlinePlus. Testosterone Levels Test. U.S. National Library of Medicine. https://medlineplus.gov/lab-tests/testosterone-levels-test/
- Nassar GN, Leslie SW. Physiology, Testosterone. StatPearls. NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK526128/
- Winters SJ. Laboratory Assessment of Testicular Function. Endotext. NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK279145/
- Braunstein GD, Reitz RE, Buch A, Schnell D, Caulfield MP. Testosterone reference ranges in normally cycling healthy premenopausal women. J Sex Med. 2011. https://pubmed.ncbi.nlm.nih.gov/21771278/