This test measures levels of free (unbound) T3, a thyroid hormone that is produced by the thyroid gland and converted from the thyroid and plays an important role in the body’s metabolism.
Most of your T3 is attached to proteins. The part that is not bound to proteins is known as free T3 (FT3). This is the part that is tested here. T3, both total and free, is measured to detect a problem with your thyroid hormones.
Read more about the Free T-3 Blood Test