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Test Code T4FRE T4 Free

Additional Codes

Epic: LAB127

Specimen Required

Collection Container/Tube
Preferred: Serum gel
Acceptable: Plasma Lithium Heparin
Submission Container/Tube: Plastic vial
Specimen Volume: 3.0 mL
Collection Instructions: Centrifuge and separate cells after clot formation and within 4 hours of collection.  Remove serum if testing will be delayed more than 24 hours.

Specimen Minimum Volume

0.25 mL

Specimen Stability

Room temperature: Unacceptable
Refrigerated on gel: 24 hours
Refrigerated off gel:  6 days
Frozen: >6 days

Reference Range

0.70 - 1.48 ng/dL

Rejection Due To

Samples stored on gel >24 hours, unlabeled, mislabeled, wrong tube type, QNS, severely hemolyzed samples, and any sample from which the lipid layer has not been removed.

Report Available

Same day

Clinical Significance

Thyroxine (T4) circulates in the blood as an equilibrium mixture of free and serum protein bound hormone.  Thyroxine binding globulin (TBG), albumin, and pre-albumin bind approximately 75%, 10%, and 15% of the total circulating T4 respectively.  The binding of T4 by these proteins is such that less than 0.03% is present in the circulation as unbound, Free T4.  This small percentage of the total T4 represents the physiologically available hormone which is biologically active.  Once the Free T4 is absorbed by the target cells, the equilibrium reestablishes circulating free T4 levels.   The equilibrium establishes constant levels of Free T4, thus assuring target tissues of receiving the required amount of hormone.  Free T4 values may be the best indicator of thyroid dysfunction, since Free T4 is less sensitive to changes in serum binding proteins.

Method Name

Chemiluminescent Microparticle Immunoassay (CMIA).


Free T4 assay is designed to have a mean potential interference from hemoglobin, bilirubin, triglycerides, and protein of < 10% at the levels indicated below:


  Hemoglobin -     ≤ 500 mg/dL

  Bilirubin -            ≤ 20 mg/dL

  Triglycerides -    ≤ 3000 mg/dL

  Protein -             ≤ 12 g/dL


NOTEPerformance of this test has not been established with neonatal specimens.



Day(s) Performed