This is for informational purposes only. For medical advice or diagnosis, consult a professional. Free T3 (Free Triiodothyronine) is a blood test that measures the level of unbound T3 hormone in your blood. T3 is one of the two main hormones produced by the thyroid gland, with T4 being the other. "Free" T3 refers to the T3 that is not attached to proteins in the blood and is therefore available to enter body tissues and have an effect. This test is often used along with TSH and Free T4 tests to: Diagnose thyroid disorders: Including hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid). Monitor thyroid treatment: Especially when TSH and Free T4 levels are not giving a clear picture, or in cases of T3 thyrotoxicosis.