Free Thyroxine
Label Mnemonic: | FT4 |
Epic code: | LAB127 |
Downtime form: | A-1a Doctor/Provider Orders - Pathology Core and Specialty Care Nursery |
Chemistry
6240 RCP
356-3527
6240 RCP
356-3527
Specimen(s):
Plasma
Collection Medium:
Plasma Separator Tube 4.5 mL |
Alternate
Collection Media:
Call laboratory for additional acceptable specimen collection containers.
Minimum:
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
Comments:
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 increased 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).
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 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.
Methodology:
Electrochemiluminescence Immunoassay
CPT Code:
84439
See Additional Information:
Biotin Interference with Immunoassays
Biotin Interference with Immunoassays