Free T4 is the active form of thyroxine. This test differs from a Total T4 test in that it isn't affected by the level of protein in the blood. Free T4 can also help to diagnose Hypothyroidism and Hyperthyroidism and also helps to determine the cause of these conditions. If you seem to never be able to lose weight no matter what you do, you would want to order this test. The opposite is true if you seem to not be able to gain weight.