Specimen

Plasma

Collection Medium:

Alternate
Collection Media:

Call laboratory for additional acceptable specimen collection 
containers.

Minimum:

2 mls whole blood in light green top or two 0.6 ml microtubes

Testing Schedule:

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

Analytic Time:

3 hours (upon receipt in laboratory)

Reference Range:

Normal adults: 4.6 - 12.0 mcg/dl
Pediatrics: See Appendix 'Age Variation of Serum Thyroxine'

Comments:

New analytical immunoassay with different references ranges instituted 
4/24/00 at 1000.

Test
Limitations:

The assay is unaffected by icterus (bilirubin is less than 37 mg/dl), 
hemolysis (Hb is less than 2.3 g/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).

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 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.

The test cannot be used to determine T4 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 first be interrupted 
for 4-6 weeks to allow the physiological state to become 
reestablished.*

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.

*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:

84436

See Additional Information:
Thyroxine - Age Variation in Serum