Free T3 is an important hormone in the regulation of metabolism. Free T3 test measures the amount of T3 hormone in the blood. It is measured as part of a thyroid function evaluation. Most of the thyroid hormone made in the thyroid is in the form of T4. The body's cells convert the T4 to T3, which is the more active hormone. It is the most powerful thyroid hormone, and affects almost every process in the body, including body temperature, growth, and heart rate.