Useful For

The Free T4, Thyroxine, is generally used as a second line test after the TSH to help evaluate TSH changes. This combination assists the physician in the determination of whether the thyroid hormone feedback system is functioning as it should. The Free T4 is not affected by the amount of protein available in the blood to bind to the hormone. Since Free T4 is the active form of Thyroxine, it is believed to be a more accurate reflection of thyroid hormone function than the Total T4. The combination of the Free T4 and the TSH assist the physician in the determination of the causes of hyperthyroidism or hypothyroidism as well as monitoring the treatment for both disorders.