Test Catalog

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

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

Evaluation of suspected thyroid function disorders using free thyroxine measured together with thyroid-stimulating hormone

Testing Algorithm Delineates situations when tests are added to the initial order. This includes reflex and additional tests.

See Thyroid Function Ordering Algorithm in Special Instructions.

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. 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.


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


Adult (> or =20 years): 0.9-1.7 ng/dL


For SI unit Reference Values, see International System of Units (SI) Conversion

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

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 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 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.

Clinical Reference Recommendations for in-depth reading of a clinical nature

Melmed S, Polonsky KS, Larsen PR, Kronenberg H: Williams Textbook of Endocrinology. 12th ed. Saunders Company; 2011:348-414

Special Instructions Library of PDFs including pertinent information and forms related to the test