In males, testosterone is the primary sex hormone. During puberty, testosterone causes a boy's body hair to grow, muscles to expand, and his voice to deepen. It regulates sex drive, preserves muscular mass, and aids sperm production in adult males. Women's bodies contain testosterone as well, albeit in much smaller amounts. This test determines how much testosterone is present in your blood. The majority of testosterone in the blood is bound to proteins. Free testosterone is testosterone that has not been linked to a protein.