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

Phone Numbers:

MIN Lab: 612-813-6280

STP Lab: 651-220-6550

Test Availability:

Daily, 24 hours

Turnaround Time:

1 day

Special Instructions:

N/A

Specimen

Specimen Type:

Blood

Container:

Preferred: Green top (Li Hep) tube
Alternate: 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) plasma/serum

Collection:

Routine venipuncture

Special Processing:

Lab Staff: Centrifuge specimen, remove plasma/serum aliquot into a plastic sample cup. Store at 2-8 degrees Centigrade for up to 7 days.

Patient Preparation:

None

Sample Rejection:

Mislabeled or unlabeled specimen

Interpretive

Reference Range:

0.8 - 1.8 ng/dL

Critical Values:

N/A

Limitations:

Elevated values may be seen in patients on thyroxine therapy. Heterophile antibodies may interfere with immunoassay tests. A number of drugs including phenytoin, carbamezapine, ibuprofen, and salicylate, may interfere with Free T4 results due to competition for TBG sites. Thyroid autoantibodies when present may cause elevated results.

Methodology:

Homogeneous Sequential Chemiluminescent Immunassay

References:

Siemens Dimension Vista Directions For Use (8/21/2013) Siemens Healthcare Diagnostics

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

Updates:

7/22/2014: Method change, previously listed as Competitive Immunoassay/Competitive.



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