Testosterone is a hormone primarily associated with male physiology, but it is also present in smaller amounts in females. Free testosterone refers to the amount of testosterone in the bloodstream that is not bound to a protein, which is the form that is available for the body to use. Free testosterone is an important measure because it is the biologically active form of the hormone that can interact with receptors in various tissues, such as muscle and bone, to promote growth and development. Testing for free testosterone levels can provide insight into potential hormonal imbalances or conditions, such as hypogonadism, which can affect fertility, libido, and overall health.