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Tests measuring free T4 – either a free T4 (FT4) or free T4 index (FTI) – more accurately reflect how the thyroid gland is functioning when checked with a TSH. The finding of an elevated TSH and low FT4 or FTI indicates primary hypothyroidism due to disease in the thyroid gland.
A blood test that measures both free and bound T4 is called a total T4 test. Other blood tests measure just free T4. Healthcare providers most often use a free T4 test to assess thyroid function because it’s more accurate than a total T4 test.
Free thyroxine comprises a small fraction of total thyroxine. The free T4 (FT4) is available to the tissues and is, therefore, the metabolically active fraction. Elevations in FT4 cause hyperthyroidism, while decreases cause hypothyroidism.
Total T4 measures the total amount of thyroxine in the blood, including both the free and protein-bound forms. Free T4, on the other hand, measures only the unbound, biologically active form of thyroxine.
T4 is the main form of thyroid hormone circulating in the blood. A Total T4 measures the bound and free hormone and can change when binding proteins differ (see above). A Free T4 measures what is not bound and able to enter and affect the body tissues. Tests measuring.
Free T4 index, a formula that includes total T4 and a measurement called thyroid hormone-binding index. The normal range for the free T index is 4.8 to 12.7 mcg/dL. If your results show high total T4 or a high free T4 index, it means you may have hyperthyroidism.
A total T4 test measures free and bound T4 together. A calculation may be done to figure out how much T4 is free. This is a less accurate way to check how well your thyroid is working. Other names: free thyroxine, free T4, total T4 concentration, T4 index, thyroxine screen, free T4 concentration, thyroxine test by equilibrium dialysis.