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Test Catalog

Test ID: FRT4    
T4 (Thyroxine), Free, Serum

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Useful For Suggests clinical disorders or settings where the test may be helpful

Free thyroxine is measured together with thyroid-stimulating hormone when thyroid function disorders are suspected.

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

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.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

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

Interpretation Provides information to assist in interpretation of the test results

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).

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

In patients receiving therapy with high biotin doses (ie, >5 mg/day), no sample should be taken until at least 8 hours after the last biotin administration.


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 FDH familial dysalbuminemic hyperthyroxinemia (FDH), 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 anti-animal 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 anti-mouse 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.

Clinical Reference Provides recommendations for further in-depth reading of a clinical nature

Melmed S, Polonsky KS, Larsen PR, et al: Williams textbook of Endocrinology. 12th edition. Elsevier Saunders Company, 2011, pp 348-414

Special Instructions and Forms Describes specimen collection and preparation information, test algorithms, and other information pertinent to test. Also includes pertinent information and consent forms to be used when requesting a particular test