Testosterone is carried in plasma bound to a plasma protein called sex hormone binding globulin. Most of circulating testosterone (60%) is bound to this liver-derived protein. The rest (38%) is bound to albumin. Only about 2% of circulating testosterone is free and able to enter cells and exert its metaboloc effects. Most circulating testosterone is metabolized to 17-ketosteroids in the liver and conjugated to glucuronide or sulfate prior to being excreted in the urine.