The test measures the amount of free triiodothyronine, or FT3, in your blood. T3 is one of two major hormones produced by the thyroid gland (the other hormone is called thyroxine, or T4). The thyroid gland is found in the neck, in front of the windpipe. T3 makes up less than 10% of what we call thyroid hormone, while T4 makes up the rest. T3, however, is about four times as strong as T4, and is thought to cause most, if not all,…The test measures the amount of free triiodothyronine, or FT3, in your blood. T3 is one of two major hormones produced by the thyroid gland (the other hormone is called thyroxine, or T4). The thyroid gland is found in the neck, in front of the windpipe. T3 makes up less than 10% of what we call thyroid hormone, while T4 makes up the rest. T3, however, is about four times as strong as T4, and is thought to cause most, if not all, the effects of thyroid hormones. Many of your body’s cells can turn T4 into T3; T4 may be mainly a “reservoir” used to make T3 available. Thyroid hormones help regulate the body’s metabolism (how the body functions). About 99.7% of T3 in blood is attached to a specific protein, and the rest is unattached (free). Blood tests can measure either the total (both bound and unattached) or free T3 hormone in the blood. These tests are called TT3 (total T3), and FT3 (free T3) respectively. Most laboratories will only routinely measure and report FT3 results. How is the sample collected for testing? A blood sample is obtained from a needle placed in a vein in your arm. Is any test preparation needed to ensure the quality of the sample? None needed; however, certain medications can interfere with the FT3 test, so tell your doctor about any drugs that you are taking See MoreSee Less
