Test ID: FRT4 T4 (Thyroxine), Free, Serum
Reporting Name
T4 (Thyroxine), Free, SUseful For
Evaluation of suspected thyroid function disorders using free thyroxine measured together with thyroid-stimulating hormone
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).
Testing Algorithm
See Thyroid Function Ordering Algorithm in Special Instructions.
Special Instructions
Report Available
Same day/1 to 2 daysDay(s) Performed
Monday through Sunday
Clinical Reference
Melmed S, Polonsky KS, Larsen PR, Kronenberg H: Williams Textbook of Endocrinology. 12th ed. Saunders Company; 2011:348-414
Method Name
Electrochemiluminescence Immunoassay
Specimen Type
SerumSpecimen 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
Test Classification
This test has been cleared, approved, or is exempt by the US 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 |
mml-endo-thyroid