|Plasma Separator Tube 4.5 mL
Call laboratory for additional acceptable specimen collection containers.
3 mL whole blood in light green top tube or TWO Microtainer® devices
24 hrs/day, 7 days a week, including holidays.
1 hour (upon receipt in laboratory)
< 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.
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 339 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