Thyroxine (T4) is the main hormone produced by the thyroid gland and is essential for regulating metabolism, growth, and development throughout the body. This test measures the total amount of T4 in the blood, including both the free (active) form and the protein-bound form. T4 levels help evaluate overall thyroid function and are used to diagnose thyroid disorders such as hypothyroidism (low thyroid function) and hyperthyroidism (overactive thyroid). Abnormal T4 levels can cause symptoms like fatigue, weight changes, mood disturbances, and heart problems. This test is often ordered as part of a comprehensive thyroid evaluation, along with other thyroid tests like TSH and T3. Regular monitoring of T4 levels is important for patients with thyroid disorders to ensure proper treatment and prevent complications.