Free T3 and Free T4 Blood Test Panel
Free Tri-iodothyronine (T3) Serum and Thyroxine (T4) Free Direct Serum.
A T3 normally represents only approximately 5% of the thyroid hormone and like thyroxine is almost entirely bound to the carrier proteins, with only 0.25% of the total being in the free state. Measurement of Free T-3 is of value in confirming the diagnosis of hyperthyroidism, when an elevated free or total thyroxine level is found. Abnormal total and free tri-iodothyronine concentrations can appear in T3 toxicosis, in the presence of normal thyroxine levels. Free T3 levels are not affected by carrier protein variation.
A Free T4 is the active form of thyroxine and is thought to be a more accurate reflection of thyroid hormone function. The free T4 test is thought by many to be a more accurate reflection of thyroid hormone function and, in most cases, its use has replaced that of the total T4 test. A total T4 or free T4 test is primarily ordered in response to an abnormal TSH test result. Sometimes the T4 will be ordered along with a TSH to give the doctor a more complete evaluation of the adequacy of the thyroid hormone feedback system. These tests are usually ordered when a person has symptoms of hyperthroidism or hypothyroidism.
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