A T4 test is a blood draw that is analyzed to evaluate the level of the main hormone produced by the thyroid gland, according to the National Institutes of Health. The lab test measures the amount of T4 present within the blood.
A T4 test is often performed to ensure the thyroid is functioning and to evaluate the action of thyroid hormones, explains the National Institutes of Health. If a physician suspects a patient is suffering from a thyroid disorder, such as hyperthyroidism or hypothyroidism, a T4 test shows if the thyroid is producing too much or not enough of the hormone. A normal range of T4 is 4.5 to 11.2 micrograms per deciliter.