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

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

The test generally is used as a second-line test after thyroid-stimulating hormone (TSH) to help evaluate TSH changes.


The free thyroxine value, combined with the TSH value, gives a more accurate picture of the thyroid status in patients with abnormal thyroid-binding globulin levels such as those who are pregnant or those who are receiving treatment with estrogens, androgens, phenytoin, or salicylates.

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

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.

0.8-1.8 ng/dL

Elevated values are seen in patients on thyroxine therapy.

Interpretation Provides information to assist in interpretation of the test results

Elevated values suggest hyperthyroidism or exogenous thyroxine (T4).


Decreased values suggest hypothyroidism.


Free T4 works well to correct total T4 values for thyroid-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

Heparin, free fatty acids, and autoantibodies to thyroxine (T4) can cause misleading values.


Phenytoin may interfere with free T4 levels due to competition with T4 for thyroid-binding globulin binding sites.

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

1. Klee GG, Hay ID: Biochemical thyroid function testing. Mayo Clin Proc 1994;69:469-470

2. Klee GG, Hinz VS: The Ciba Corning ACS:180 Plus. In Immunoassay Automation: An Updated Guide to Systems. San Diego, CA, Academic Press, Inc., 1996, pp 63-102

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