The Total T3 test measures the complete amount of the T3 thyroid hormone in your blood, including both 'bound' (inactive) and 'free' (active) forms. T3 is the most potent thyroid hormone that controls your metabolism. This test is a key part of a thyroid investigation, especially for diagnosing hyperthyroidism (overactive thyroid), where T3 levels are often high. A simple blood draw is required. For the most accurate picture of your usable hormone level, we recommend our Free T3 test, which is included in our comprehensive Thyroid Function Profile.