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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.

Reference Values

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

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.

CPT Information

84439: Chemistry: Thyroid Tests Thyroxine; Total free

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

LOINC Code Information

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