Test name: Free Thyroxine
Order Code: FT4
Preferred Specimen: 2mL serum in a Serum Separator Tube (SST or Tiger-top)
CPT Code: 84439
Collection: Allow SST to clot in an upright position for 30 min, then centrifuge sample within 2 hours of collection. Refrigerate.
Special Instructions and Considerations: Samples should not be taken from patients receiving therapy with high biotin doses (i.e. > 5 mg/day) until at least 8 hours following the last biotin administration. Any influence that might affect the binding behavior of the binding proteins can alter the result of the free T4 tests (e.g. drugs, NTIs (Non-Thyroid-Illness) or patients suffering from FDH (Familial Dysalbuminemic Hyperthyroxinemia) or increased TBG in pregnancy). The test cannot be used in patients receiving treatment with lipid-lowering agents containing D-T4. If the thyroid function is to be checked in such patients, the therapy should first be discontinued for 4-6 weeks to allow the physiological state to become re-established. Autoantibodies to thyroid hormones can interfere wih the assay. In in vitro studies the drugs Furosemide and Levothyroxine caused elevated free T4 findings at the daily therapeutic dosage level.
Transport Temperature: Refrigerated
Specimen Stability: 7 days refrigerated