Free Thyroxine (FT4)
Free T4
Testing Information
Sample Type
Serum
Test Codes
630230
CPT Code
84439
Testing Area
Core Lab - Chemistry
Test Methodology
Abbott Alinity - Chemiluminescence Immunoassay
Reference Range
0-10 days: 0.5-2.0 ng/dL>10 days: 0.7-1.5 ng/dL
Expected Turnaround Time
Results Available 24 hours from Receipt
Specimen Collection Criteria
Collect one gold top SST tube (minimum whole blood: 4 mL). Let specimen clot for 30-60 minutes and then centrifuge to separate serum from cells. Refrigerate after centrifugation.
Rejection Criteria
Grossly hemolyzed specimens, quantity not sufficient, improperly labeled
Specimen Validity and Retention
Centrifuged SST Tubes:Room Temperature (20-25°C): 2-4 hoursRefrigerated (2-8°C): 7 daysFrozen (-20°C or greater): UnacceptableSerum Aliquots:Room Temperature (20-25°C): 2-4 hoursRefrigerated (2-8°C): 7 daysFrozen (-20°C or greater): 7 daysSpecimens retained for 7 days after testing
Courier Transportation
Transport centrifuged tube refrigerated (2-8°C)
Clinical Utility
Free T4 assays are used when TSH results are not consistent with the patient"s clinical picture, especially in those with euthyroid but elevated TBG.
Special Notes
Stop Biotin supplements? Phenytoin interference.
