This is the 'accelerator' test for your body's metabolism. The Free T3 test measures the active, usable form of T3, the thyroid hormone that directly controls your energy and metabolism. While a TSH test is a great start, this test shows what's actually available for your cells to use. It is the most accurate test for diagnosing and managing hyperthyroidism (an overactive thyroid) and for fine-tuning your treatment plan. A simple blood draw is required. This test provides a clear look at your true active hormone level, helping you and your doctor get your energy back in balance.