Free Thyroxine
Label Mnemonic: FT4
Epic Lab Code: LAB127
Downtime Form: A-1a General Laboratory Requisition
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
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). In patients receiving therapy with high biotin doses (i.e. is greater than 5 mg/day) no sample should be taken until at least 8 hours after the last biotin administration. 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 re-established.* 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 1984;3:475-481.
Electrochemiluminescence Immunoassay
CPT Code: