The typical range for free T4, or free thyroxine, in a thyroid test is 0.7 to 1.9 ng/dl, according to EndocrineWeb. Typical serum thyroxine, or T4, ranges from 4.6 to 12 ug/dl.
A blood sample is needed to check levels of T4, which is the main hormone that the thyroid gland produces, notes MedlinePlus. The test is generally performed to test thyroid function when signs of a thyroid disorder are present; it can be useful in diagnosing or ruling out hyperthyroidism, hypothyroidism, hypopituitarism, thyroid nodules or thyrotoxic periodic paralysis. High T4 levels can indicate Graves' disease, thyroiditis and germ cell tumors, among other conditions, while low levels can indicate malnutrition or Hashimotos disease.