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Lab Dept:
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Chemistry
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Test Name:
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FREE THYROXINE (T4)
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General Information
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Lab Order Codes:
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FT4
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Synonyms:
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Free T4
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CPT Codes:
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84439 – Thyroxine; free
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Test Includes:
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Free thyroxine concentration reported in ng/dL.
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Logistics
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Test Indications:
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Free Thyroxine (T4) is the portion of T4 that is not bound to transport proteins. It is metabolically active and is a precursor to triiodothyronine (T3). Free T4 levels correlate with T4 secretion and metabolism.
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Lab Testing Sections:
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Chemistry (all specimens tested on the St. Paul campus)
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Phone Numbers:
Minneapolis:
Saint Paul:
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612-813-6280
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651-220-6550
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Test Availability:
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Daily, 24 hours
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Turnaround Time:
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1 – 3 days, test performed Monday – Friday.
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Special Instructions:
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N/A
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Specimen
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Specimen Type:
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Blood
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Container:
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Red top (plain, no gel) tube
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Draw Volume:
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1.5 mL (Minimum: 0.6 mL) blood
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Processed Volume:
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0.5 mL (Minimum: 0.2 mL) serum
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Collection:
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Routine venipuncture
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Special Processing:
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Lab Staff: Centrifuge specimen, remove serum aliquot into a plastic sample cup. Store at refrigerated temperatures for up to 48 hours. Store at –20°C if testing is delayed for more than 48 hours.
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Patient Preparation:
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None
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Sample Rejection:
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Mislabeled or unlabeled specimen, specimens other than serum, gross hemolysis
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Interpretive
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Reference Range:
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0.8 - 1.8 ng/dL
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Critical Values:
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N/A
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Limitations:
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Elevated values are seen in patients on thyroxine therapy. Heterophile antibodies may interfere with immunoassay tests. The drug phenytoin may interfere with Free T4 results due to competition for TBG sites. Thyroid autoantibodies when present may cause elevated results.
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Methodology:
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Competitive Immunoassay/Chemiluminescence
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References:
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DPC End User Procedure Manual Disk #0408111 (2005)
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Siemens Immulite Free T4 product insert, PIL2KF4-15, 08/13/2008
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National Committee for Clinical Laboratory Standards (NCCLS) (2002) Clinical Laboratory Technical Procedure Manuals; Approved Guideline-Fourth Edition GP2-A4
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Biorad Immunoassay Plus Control Product Insert
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Burtis, CA, Ashwood, ER, Bruns, DE, ed (2006) Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 4th Edition, Philadelphia: W.B. Saunders
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Jacobs and DeMott Laboratory Test handbook (2001) 5th Edition, Lexi-comp Inc
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