A normal T4 range is 4.5 to 11.2 micrograms per deciliter of blood, according to MedlinePlus. The T4 test measures the amount of thyroxine produced by the thyroid gland.
A higher than normal level of T4 in the blood may indicate the presence of conditions related to an overactive thyroid, including Graves disease, subacute thyroiditis, toxic multinodular goiter and germ cell tumors, reports MedlinePlus. Trophoblastic disease and over-treatment with thyroid hormone replacement medicine may also cause elevated T4 levels.
A lower than normal level of T4 may indicate an underactive thyroid or hypothyroidism, which includes Hashimoto disease and other conditions, according to MedlinePlus. Low T4 levels also result from the use of certain medications, malnutrition, fasting and illness.