Clinics and Departments

Laboratory Services

Lab Dept:

Chemistry

Test Name:

FREE THYROXINE (T4)

General Information

Lab Order Codes:

FT4

Synonyms:

Free T4

CPT Codes:

84439 – Thyroxine; free

Test Includes:

Free thyroxine concentration reported in ng/dL.

Logistics

Test Indications:

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.

Lab Testing Sections:

Chemistry (all specimens tested on the St. Paul campus)

Phone Numbers:

MIN Lab: 612-813-6280

STP Lab: 651-220-6550

Test Availability:

Daily, 24 hours

Turnaround Time:

1 – 3 days, test performed Monday – Friday.

Special Instructions:

N/A

Specimen

Specimen Type:

Blood

Container:

Red top (plain, no gel) tube

Draw Volume:

1.5 mL (Minimum: 0.6 mL) blood

Processed Volume:

0.5 mL (Minimum: 0.2 mL) serum

Collection:

Routine venipuncture

Special Processing:

Lab Staff: Centrifuge specimen, remove serum aliquot into a plastic sample cup. Store at refrigerated temperatures for up to 48 hours. Store at –20C if testing is delayed for more than 48 hours.

Patient Preparation:

None

Sample Rejection:

Mislabeled or unlabeled specimen; specimens other than serum; gross hemolysis

Interpretive

Reference Range:

0.8 - 1.8 ng/dL

Critical Values:

N/A

Limitations:

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.

Methodology:

Competitive Immunoassay/Chemiluminescence

References:

DPC End User Procedure Manual Disk #0408111 (2005)

Siemens Immulite Free T4 product insert, PIL2KF4-15, 08/13/2008

National Committee for Clinical Laboratory Standards (NCCLS) (2002) Clinical Laboratory Technical Procedure Manuals; Approved Guideline-Fourth Edition GP2-A4

Biorad Immunoassay Plus Control Product Insert

Burtis, CA, Ashwood, ER, Bruns, DE, ed (2006) Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 4th Edition, Philadelphia: W.B. Saunders

Jacobs and DeMott Laboratory Test handbook (2001) 5th Edition, Lexi-comp Inc



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