This test measures free thyroxine, or FT4, in your blood. Thyroxine is one of two major hormones produced by the thyroid gland (the other is called triiodothyronine (T3)). Within the blood, most thyroid hormones are attached to a specific protein, but it is the hormones that are free from these proteins that are able to affect body functions. This is therefore why we measure the ‘free’ thyroxine in your blood sample.&…This test measures free thyroxine, or FT4, in your blood. Thyroxine is one of two major hormones produced by the thyroid gland (the other is called triiodothyronine (T3)). Within the blood, most thyroid hormones are attached to a specific protein, but it is the hormones that are free from these proteins that are able to affect body functions. This is therefore why we measure the ‘free’ thyroxine in your blood sample. The thyroid gland is found in the neck, in front of the windpipe. Thyroxine (T4) makes up nearly all of what we call thyroid hormone, while triiodothyronine (T3) makes up less than 10%. Thyroid hormones help regulate the body’s metabolism (that is, how the body functions). Most T4 in blood is attached to a specific protein; less than 1% is unattached. Blood tests can measure either the total (both bound and unattached, TT4) or free (unattached, FT4) T4 hormone in your blood. Most laboratories measure FT4, as it is the free hormone that is thought to be responsible for all the effects of thyroid hormone within the body. 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? No test preparation is necessary. See MoreSee Less
