T3, or triiodothyronine, is a hormone that helps control many body functions, including growth, body temperature, and heart rate. It is one of the two major hormones made by the thyroid gland, the other being thyroxine (T4) . T3 is the active form of thyroid hormone, meaning it impacts cells in your body, whereas T4 is the inactive form of thyroid hormone. Most of the T3 in your blood is from your body’s conversion of T4 into T3 outside of your thyroid gland.

A T3 test measures the level of triiodothyronine (T3) in your blood and is most often used to diagnose hyperthyroidism, a condition in which the body makes too much thyroid hormone. The test can also help diagnose other thyroid conditions and is often ordered alongside a T4 (thyroxine) test and a TSH (thyroid-stimulating hormone) test. The T3 test measures both bound and free T3, with the total T3 test measuring the T3 that is both attached to proteins and floating free in the blood, and the free T3 test measuring only the T3 that is floating free in the blood.