Free T4 is a blood test that measures the amount of thyroxine that is not attached to a protein in the blood. Thyroxine, or T4, is the main hormone produced by the thyroid gland, and it has two forms: free T4 and bound T4. Free T4 is the active form of thyroxine hormone that enters your tissues where its needed, while bound T4 attaches to certain proteins which prevent it from entering your tissues. A free T4 test is used to find out how well your thyroid gland is working, and it can help diagnose and monitor thyroid disease and conditions that may affect your thyroid, including hypothyroidism and hyperthyroidism. The test is done by taking a blood sample, and the normal range for free T4 in adults is 0.8 to 1.8 nanograms per deciliter (ng/dL) . Test results may be affected by age, gender, health history, and other factors, and abnormal results may require further testing.