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Free thyroxine (FT4) comprises a small fraction of total thyroxine. The FT4 is available to the tissues and is, therefore, the metabolically active fraction.
Elevations in FT4 cause hyperthyroidism, while decreases cause hypothyroidism.
Free thyroxine is measured together with thyroid-stimulating hormone when thyroid function disorders are suspected.
Elevated values suggest hyperthyroidism or exogenous thyroxine.
Decreased values suggest hypothyroidism.
Free thyroxine (FT4) works well to correct total T4 values for thyroxine-binding globulin alterations, but may give misleading values when abnormal binding proteins are present or the patient has other major illnesses (euthyroid sick syndrome).
Twelve hours before this blood test do not take multivitamins or dietary supplements containing biotin or vitamin B7, which are commonly found in hair, skin, and nail supplements and multivitamins.
Of 26 commonly used pharmaceuticals tested in vitro, only furosemide caused elevated free thyroxine (FT4) findings at the daily therapeutic dosage level.
The test cannot be used in patients receiving treatment with lipid-lowering agents containing dextrothyroxine (D-T4). If the thyroid function is to be checked in such patients, the therapy should first be discontinued for 4 to 6 weeks to allow the physiological state to become re-established.
Binding protein anomalies seen with familial dysalbuminemic hyperthyroxinemia, for example, may cause values which, while characteristic of the condition, deviate from the expected results.
For assays employing antibodies, the possibility exists for interference by human antianimal antibodies (ie, heterophile antibodies) in the patient sample. Patients who have been regularly exposed to animals or have received immunotherapy or diagnostic procedures utilizing immunoglobulins or immunoglobulin fragments may produce antibodies, eg, human antimouse antibodies (HAMA), that interfere with immunoassays. This may falsely elevate or falsely decrease the results.
Interference due to extremely high titers of antibodies to analyte-specific antibodies, streptavidin or ruthenium can occur.
Adult (> or =20 years): 0.9-1.7 ng/dL
0-5 days: 0.9-2.5 ng/dL
6 days-2 months: 0.9-2.2 ng/dL
3-11 months: 0.9-2.0 ng/dL
1-5 years: 1.0-1.8 ng/dL
6-10 years: 1.0-1.7 ng/dL
11-19 years: 1.0-1.6 ng/dL
Melmed S, Polonsky KS, Larsen PR, et al: Williams textbook of Endocrinology. 12th edition. Elsevier Saunders Company, 2011, pp 348-414