A T4 test measures the level of thyroxine, a major hormone produced by the thyroid gland. T4 plays a key role in controlling metabolism, energy production, body temperature, and overall growth and development. Abnormal T4 levels can indicate hyperthyroidism (high T4) or hypothyroidism (low T4). Doctors usually recommend this test when symptoms such as weight changes, fatigue, hair fall, anxiety, dry skin, or changes in heart rate suggest a thyroid disorder.

For the T4 test, a small blood sample is collected from a vein, and no fasting or special preparation is required. The test is often performed together with T3 and TSH to provide a complete picture of thyroid function. These combined results help diagnose thyroid diseases, monitor treatment, and assess hormonal balance.