The FT4 (Free Thyroxine) test measures the level of free T4, the unbound and active form of the thyroid hormone thyroxine circulating in the blood. T4 is the primary hormone produced and secreted by the thyroid gland. Most T4 is bound to proteins in the blood, while a small fraction (free T4) is biologically active and available to the body’s tissues.
This test is commonly used to assess thyroid gland function and is one of the most important tests in diagnosing thyroid disorders. It is typically ordered alongside TSH (Thyroid Stimulating Hormone) to diagnose hypothyroidism or hyperthyroidism, monitor patients on thyroid hormone replacement therapy or antithyroid medications, and evaluate pituitary gland disorders that affect thyroid function.
