The typical normal range of T4 levels is 4.5 to 11.2 micrograms per deciliter, states MedlinePlus. Normal range may vary from one laboratory to another due to different measurements and specimens.
T4, or thyroxine, is a hormone produced by the thyroid gland. A person undergoes a T4 test to measure thyroxine levels, which help to determine thyroid function. A doctor may recommend the test if a patient shows symptoms of thyroid conditions such as hyperthyroidism, hypothyroidism, hypopituitarism and thyrotoxic periodic paralysis, according to MedlinePlus. Elevated T4 levels indicate an overactive thyroid, which results from germ cell tumors, Graves disease, subacute thyroiditis or trophoblastic disease. A lower T4 level may be due to malnutrition, illness or hypothyroidism.