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Test ID FRT4 T4 (Thyroxine), Free, Serum

Reporting Name

T4 (Thyroxine), Free, S

Useful For

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

Testing Algorithm

See Thyroid Function Ordering Algorithm in Special Instructions.

Specimen Type

Serum


Specimen Required


Patient Preparation: For 12 hours before specimen collection do not take multivitamins or dietary supplements containing biotin (vitamin B7), which is commonly found in hair, skin, and nail supplements and multivitamins.

Collection Container/Tube:

Preferred: Serum gel

Acceptable: Red top

Submission Container/Tube: Plastic vial

Specimen Volume: 1 mL


Specimen Minimum Volume

0.5 mL

Specimen Stability Information

Specimen Type Temperature Time Special Container
Serum Refrigerated (preferred) 14 days
  Frozen  30 days
  Ambient  72 hours

Reference Values

Pediatric

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 (≥20 years): 0.9-1.7 ng/dL

 

For SI unit Reference Values, see https://www.mayocliniclabs.com/order-tests/si-unit-conversion.html

Day(s) and Time(s) Performed

Monday through Sunday; Continuously

Test Classification

This test has been cleared, approved or is exempt by the U.S. Food and Drug Administration and is used per manufacturer's instructions. Performance characteristics were verified by Mayo Clinic in a manner consistent with CLIA requirements.

CPT Code Information

84439

LOINC Code Information

Test ID Test Order Name Order LOINC Value
FRT4 T4 (Thyroxine), Free, S 83122-2

 

Result ID Test Result Name Result LOINC Value
FRT4 T4 (Thyroxine), Free, S 83122-2

Clinical Information

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.

Interpretation

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).

Clinical Reference

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

Analytic Time

Same day/1 day

Method Name

Electrochemiluminescence Immunoassay