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Test Code T4FRER T4, Free LAB127

Important Note

Human Anti-Mouse Antibodies (HAMA) may be present in samples from patients who have received immunotherapy utilizing monoclonal antibodies. Additionally, other heterophile antibodies such as human anti-goat antibodies may be present in patient samples. Manufacturers formulate assays to minimize the effects of these antibodies; however careful evaluation of patient results must be done when patients are known to have such antibodies.


Testing is performed Monday through Sunday.

Routine: Typically completed within 4 hours after the specimen arrives at the testing site.

STAT: Typically completed within 1.5 hours after the specimen arrives at the testing site.

Useful For

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.

Specimen Requirement

0.5 mL serum or plasma is required.

Collect blood in a lithium heparin, green-top or red-top tube. PST and SST (gel tubes) are acceptable. Serum or plasma should be separated from contact with the cells within 2 hours of collection. Specimens not centrifuged within 4 hours of collection may be rejected. Refrigerate the specimen if unable to assay within 8 hours of collection.


Two-Site Immunoassay; Beckman

Reference Values

Reference range established for patients > 18 yrs:
0.61-1.12 ng/dL

CPT Information

84439: Chemistry: Thyroid Tests Thyroxine; Total free

LOINC Code Information

3024-7 Thyroxine (T4) Free [Mass/Volume] In Serum Or Plasma