A serum free testosterone test measures the level of the portion of total testosterone in the blood that is not bound to transport proteins (such as sex hormone-binding globulin (SHBG) and albumin). This "free" portion is the biologically active form, capable of crossing cell membranes and binding to androgen receptors to exert its physiological effects (fertility, libido, muscle building). It is particularly useful when there are discrepancies between clinical symptoms and total testosterone levels or when there is doubt about SHBG levels.
