Free testosterone is the portion of the hormone that is not bound to proteins such as SHBG or albumin and can thus become directly effective in the body. It constitutes only a small fraction of the total testosterone but is crucial for biological effects, such as on muscle strength, bone stability, sexual function, and mood.
If SHBG is significantly increased or decreased, the total testosterone value can give a misleading picture. In such cases, free testosterone is calculated to better assess whether there is indeed a hormone deficiency or excess.