T4 Free

Test Code

T4FRE

Alias/See Also

Epic: LAB127

Free T4 (Thyroxine)
Free Thyroxine
Thyroxine, Free

Preferred Specimen

Specimen Type: Serum
Collection Container: Serum gel
Specimen Volume: 3 mL

Minimum Volume

0.25 mL

Other Acceptable Specimens

Specimen Type: Plasma
Collection Container: Green top (Lithium heparin)

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.

Transport Container

Plastic vial

Specimen Stability

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

Reject Criteria (Eg, hemolysis? Lipemia? Thaw/Other?)

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.

Methodology

Chemiluminescent Microparticle Immunoassay (CMIA).

Setup Schedule

Daily

Report Available

Same day

Limitations

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

NOTE: Performance of this test has not been established with neonatal specimens.

Reference Range

0.70 - 1.48 ng/dL

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.

Performing Laboratory

Inova Laboratories

2832 Juniper Street

Fairfax, VA 22031

Last Updated: March 9, 2023
Last Review: N. Wolford, March 9, 2023

The CPT Codes provided in this document are based on AMA guidelines and are for informational purposes only. CPT coding is the sole responsibility of the billing party. Please direct any questions regarding coding to the payor being billed. Any Profile/panel component may be ordered separately. Reflex tests are performed at an additional charge.