Free T4 (Free Thyroxine) is a blood test that measures the level of unbound T4 hormone in your blood. T4 is one of the main hormones produced by the thyroid gland. "Free" T4 refers to the T4 that is not attached to proteins in the blood and is therefore available to enter body tissues. This test is often used along with a TSH test to: Diagnose thyroid disorders: Including hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid). Monitor thyroid treatment: Especially when TSH levels are not giving a clear picture.