Free Thyroxine
Label Mnemonic: FT4
Epic code: LAB127
Downtime form: A-1a Doctor/Provider Orders - Pathology Core and Specialty Care Nursery
6240 RCP
Collection Medium:
Plasma Separator Tube 4.5 mL
Alternate Collection Media:
Call laboratory for additional acceptable specimen collection containers.
3 mL whole blood in light green top tube or TWO Microtainer® devices
Testing Schedule:
24 hrs/day, 7 days a week, including holidays.
Turn Around Time:
1 hour (upon receipt in laboratory)
Reference Range:
AGE                      MALES/FEMALES
< 12 months              0.8-2.7 ng/dL
1 - 5 years              0.8-1.75 ng/dL
5 years - 18 years       0.90-1.70 ng/dL
18+ years                0.80-1.80 ng/dL
This assay may be significantly impacted by high-dose biotin (>5 mg dose) taken within previous 12 hours. High concentrations of biotin may lead to falsely decreased results. These concentrations may be found in patients taking over-the-counter supplements with biotin content much higher than nutritional requirements for biotin. Specimens should not be collected until at least 12 hours after the last dose.

Reference range for 5 years to adult changed on 11/26/2013.
Reference range for 18 years and older changed on 2/23/2016.

This test is also performed in the Iowa River Landing (IRL) clinical laboratory (for specimens drawn at that site).
Test Limitations:
The assay is unaffected by icterus (bilirubin is less than 41 
mg/dL), hemolysis (Hb is less than 1000 mg/dL), lipemia (Intralipid is 
less than 2000 mg/dL) and biotin is less than 100 ng/mL (criterion: 
recovery within plus or minus 10% of initial value).

No interference was observed from rheumatoid factor (up to 1200 U/mL)
and samples from dialysis patients.

Of 26 commonly used pharmaceuticals tested in vitro, only furosemide 
caused elevated fT4 findings at the daily therapeutic dosage level.

The test cannot be used to determine fT4 in patients receiving 
treatment with lipid-lowering agents containing D-T4. If the thyroid 
function is to be checked in such patients, the therapy should be 
interrupted for 4-6 weeks to allow the physiological state to become 

Samples from neonates have not been tested with the FT4 assay.

Auto-antibodies to thyroid hormones can interfere with the assay.

The risk of interference from potential immunological interactions 
between test components and rare sera has been minimized by the 
inclusion of additives. In rare cases, interference due to extremely 
high titers of antibodies to streptavidin ruthenium can occur.

For diagnostic purposes, the FT4 findings should always be assessed in 
conjunction with the patient's medical history, clinical examination 
and other findings.

*Bantle JP, Hunninghake DB, Frantz ID, Kuba K, Mariash CN, Oppenheimer
 JH.  Comparison of Effectiveness of Thyrotropin-Suppressive Doses of
 D- and L-Thyroxine in Treatment of Hypercholesterolemia. Am J Med
Electrochemiluminescence Immunoassay
CPT Code:
See Additional Information:
Biotin Interference with Immunoassays