The T4 test measures the level of thyroxine (T4), a key hormone produced by the thyroid gland that regulates metabolism, energy production, and growth. This test is typically performed by drawing a blood sample, which is then analyzed in a laboratory to determine the amount of T4 in the bloodstream. The T4 test is used to assess thyroid function, helping diagnose conditions such as hypothyroidism (low thyroid hormone) or hyperthyroidism (high thyroid hormone). It is often administered when symptoms such as fatigue, weight changes, heat intolerance, or swelling in the neck (goiter) are present. The test may also be used for routine screening in individuals with a history of thyroid issues or during the management and monitoring of thyroid disease, especially for those already undergoing treatment for thyroid disorders.