Free Thyroxine (T4) is the biologically active form of the thyroid hormone that is not bound to proteins in the blood. This test measures the amount of free T4 directly, providing a more accurate assessment of thyroid function than total T4 measurements. Free T4 levels are not affected by changes in thyroid-binding proteins, making this test particularly useful when evaluating patients with conditions that alter protein levels, such as pregnancy, liver disease, or certain medications. Free T4 is essential for regulating metabolism, growth, and development throughout the body. Abnormal levels can indicate thyroid disorders like hypothyroidism or hyperthyroidism. This test is commonly ordered when other thyroid tests show conflicting results or when a more precise evaluation of thyroid function is needed. Free T4 measurements are crucial for accurate diagnosis and monitoring of thyroid conditions.