The T3 test also known as a free or total triiodothyronine is used to asses the thyroid function. It tests for T3 and T4 (thyroxine) hormones produced by the thyroid gland. These hormones help control the metabolism of the body. T4 is primarily secreted by thyroid stimulating hormone (TSH) and T4 is converted into T3 by the liver. Most of the T4 and T3 hormones are bound to protein in the blood. The blood test can measure total T4 (unbounded and bounded), free T4, total T3 or free T3. This test helps in diagnosis of hyperthyroidism and can also help monitor patients with thyroid disorder.