Specimen

Plasma

Collection Medium:

Alternate
Collection Media:

Call laboratory for additional acceptable specimen collection 
containers.

Minimum:

2 mL whole blood in light green top tube or TWO 0.6 mL microtubes

Testing Schedule:

24 hrs/day, 7 days a week, including holidays.

Analytic Time:

2 hours (upon receipt in laboratory)

Reference Range:

Adult:  0.93 - 1.70 ng/dl

Comments:

New analytical immunoassay with different reference range instituted 
4/24/00 at 1000.

Test
Limitations:

The assay is unaffected by icterus (bilirubin is less than 41 mg/dl), 
hemolysis (Hb is less than 2 g/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 
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