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Test Code 380 Free T4 (Thyroxine)

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.

Access Total T4: potential of falsely elevated results when biotin concentrations are > 10 ng/mL.

Additional Codes

ALAB: T4F

PowerChart: T4 (Thyroxine) Free or Free T4 (Thyroxine)

MA Cerner: T4 Free

Test Method

MercyOne DBQ: Two-Site Immunoassay; Beckman DxC700AU

MA: Immunoenzymatic ("Sandwich") Chemiluminescent Method; Roche Cobas 6000

Specimen Requirement

0.5 mL serum or plasma is required.

Additional Specimen Collection Information

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.

Performance

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

CPT Code Information

84439: Chemistry: Thyroid Tests Thyroxine; Total free

NCD Frequency Limitations: Testing may be covered up to two times a year in clinically stable patients; more frequent testing may be reasonable and necessary for patients whose thyroid therapy has been altered or in whom symptoms or signs of hyper or hypothyroidism are noted.

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.

LOINC Code Information

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