Free T4

T4, also called thyroxine, is the main form of thyroid hormone made by the thyroid gland. Most T4 is bound to proteins, while a small proportion is unbound or free. Free T4 testing measures T4 that circulates through the blood and is available to enter body tissues and act upon them. Total T4 testing measures the total amount of the hormone, including free T4 and the part that is bound to proteins.

T4 is measured by taking a blood sample. A low T4 test result may indicate an underactive thyroid gland or problems with its stimulation by the pituitary gland. And a high level of T4 may be a sign of an overactive thyroid, called hyperthyroidism. Your doctor may order a T4 test if thyroid problems are suspected or to follow up after an abnormal thyroid-stimulating hormone (TSH) test result.