The T4 blood test measures the level of thyroxine, a hormone produced by the thyroid gland, which plays a vital role in metabolism, energy regulation, and overall bodily functions. The T4 normal range for adults typically falls between 5.0 to 12.0 micrograms per deciliter (mcg/dL) for total T4 and 0.8 to 2.8 nanograms per deciliter (ng/dL) for free T4. Abnormal T4 levels can indicate thyroid disorders. Higher levels may suggest hyperthyroidism (overactive thyroid), while lower levels may point to hypothyroidism (underactive thyroid). The thyroid T4 normal range is an essential parameter for diagnosing and managing thyroid-related conditions.