Thyroxine (T-4)
Label Mnemonic: T4
Epic Lab Code: LAB126
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 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 0 - 3 days 8.0-20.0 μg/dL 3 - 30 days 5.0-15.0 μg/dL 31 days - 1 years 6.0-14.0 μg/dL 1 - 5 years 4.5-11.0 μg/dL 6 - 18 years 4.5-10.0 μg/dL Normal adults 4.6 12.0 μg/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.
Test Limitations:
The assay is unaffected by icterus (bilirubin is less than 37 mg/dL), hemolysis (Hb is less than 2300 mg/dL), lipemia (triglycerides is less than 2500 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 2400 U/mL) and samples from dialysis patients. In vitro tests were performed on 15 commonly used pharmaceuticals. No interference with the assay was found. Autoantibodies to thyroid hormones can interfere with the assay. The risk of interference from potential immunological interactions between test components and rare plasma has been minimized by the inclusion of additives. In rare cases, interference due to extremely high titers of antibodies to ruthenium can occur. T4 reagent contains additives which minimize these effects. Extremely high titers of antibodies to streptavidin can occur in isolated cases and cause interference. For diagnostic purposes, the T4 findings should always be assessed in conjunction with the patient's medical history, clinical examination and other findings.
Electrochemiluminescence Immunoassay
CPT Code:
See Additional Information:
Biotin Interference with Immunoassays