Free testosterone refers to the portion of testosterone circulating in the bloodstream that is not attached to any proteins, making it bioavailable and active at cellular levels. Testosterone in the blood exists in two main forms: bound and unbound (free). The majority of testosterone is bound to sex hormone-binding globulin (SHBG) and, to a lesser extent, albumin. Only a small fraction, approximately 2-3%, remains unbound or free, and thus, readily available to interact with androgen receptors in various tissues throughout the body. Free testosterone is crucial for many physiological processes, including muscle growth, bone density, red blood cell production, and the regulation of mood and libido. The levels of free testosterone can provide important diagnostic information, as they may more accurately reflect the hormone's active effects in the body than total testosterone levels alone. Conditions such as obesity, thyroid disorders, and aging can alter SHBG levels, subsequently affecting the amount of free testosterone. Measuring free testosterone levels can be particularly important in evaluating hypogonadism and determining the need for testosterone replacement therapy (TRT), especially in cases where total testosterone levels appear normal but symptoms of testosterone deficiency persist.