Free testosterone, also called unbound testosterone, refers to the amount of testosterone available for your body to use (rather than the total amount in your body). Testosterone governs many essential functions, such as sex drive, energy levels, healthy bones, healthy brain, muscle mass, strength, and mood. Your Free Testosterone levels may fall as you age, so low levels can be a sign that you’re peri-menopausal. If you have low Free testosterone level you have a higher chance of developing osteoporosis, neurodegenerative diseases, cardiovascular disease and skin aging.