Testosterone, Free
Free testosterone is the small fraction of testosterone not bound to proteins, meaning it is available for your tissues to use. It is often measured or calculated when total testosterone results are ambiguous.
Equilibrium dialysis is gold standard; calculated free T acceptable when SHBG measured. Method-dependent reference intervals common.
Because most testosterone travels bound to sex hormone binding globulin and albumin, the free portion can give a more accurate sense of active androgen exposure, especially when protein levels are unusual. This makes it useful in conditions where binding proteins are shifted, such as aging, obesity, thyroid disease, or pregnancy.
Elevated free testosterone may point toward increased androgen activity, sometimes seen in PCOS or with low binding-protein levels, and occasionally with supplementation. Because calculation methods and assays vary, results should be interpreted cautiously and alongside total testosterone and SHBG.
Low free testosterone can accompany aging, pituitary or gonadal changes, chronic illness, or high binding-protein levels, and its significance depends heavily on your sex and clinical context.
Free testosterone ranges are highly sex- and age-specific and depend on the assay or equation used, so values from different labs are not always comparable. A clinician should interpret your result in the context of total testosterone and SHBG.
Sources
Educational context only, not medical advice or a diagnosis. Reference ranges vary by lab, age and sex; always discuss your results with a clinician.
Associations to explore with a clinician, not a diagnosis from a single number.