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 T4, including free T4 and T4 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. A high level of T4 may be a sign of an overactive thyroid, called hyperthyroidism.