Free thyroxine (FT4) is T4 that is not bound to proteins in the blood; as such, this blood test is not affected by protein levels and is therefore a more reliable measurement of T4 levels in the blood.

Thyroxine, also know as T4, is one of the two main thyroid hormones. T4 affects nearly every physiological process in the body including regulating heart rate, metabolism and the body's temperature.

The Free T4 test is used to assess the thyroid's function and to help diagnose diseases or disorders of the thyroid like hyperthyroidism and hypothyroidism. Hashimoto's thyroiditis is the most common cause of hypothyroidism, and is often tested for with numerous thyroids tests, including TSH, Free T4 and Anti-Thyroid Antibody tests.