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Test Code Billings Clinic: 8103 T4 (Thyroxine), Free, Plasma and Serum

Important Note

Results from patients taking biotin supplements or receiving high-dose biotin therapy should be interpreted with caution due to possible interference with this test.

Performing Laboratory

Billings Clinic Laboratory: Chemistry

Useful For

Free thyroxine is measured together with thyroid-stimulating hormone when thyroid function disorders are suspected.

Methodology

Direct Chemiluminescence

Specimen Requirements

Submit only 1 of the following specimens:

Serum or plasma should be physically separated from cells as soon as possible with a maximum limit of two hours from the time of collection.

 

Plasma

  • Draw blood in a green-top (lithium heparin) tube(s).
  • Spin down and send 1.0 mL of heparinized plasma frozen.

 

Serum

  • Draw blood in a plain, red-top tube(s) or a serum gel tube(s).
  • Spin down and send 1.0 mL of serum frozen

Storage/Stability

Specimen Type Temperature Time
Plasma, Serum Refrigerated 2 days
  Frozen (preferred) 1 month
  Ambient 8 hours

 

Reject Due To

  • Gross hemolysis
  • Specimens with apparent contamination
  • Specimens stored at room temperature greater than 8 hours
  • Samples frozen and thawed more than once

Reference Values

0 - 1 day:  2.00 - 5.00 ng/mL

1 - 3 days:  0.9 - 2.20 ng/mL

3 days - 1 month:  0.8 - 2.00 ng/mL

> 1 month:  0.76 - 1.46 ng/mL

Day(s) Test Set Up

Daily

Test Classification and CPT Coding

84439 - Thyroxine; free

 

LCD or NCD test. ICD-10 code is required for this test. When appropriate, obtain a properly executed ABN and submit the ABN with test order(s). See “Medical Necessity and Advanced Beneficiary Notice (ABN) Policy and Form” under Resources for a copy of a form and additional information.

LOINC Code Information

Reporting Name LOINC Code
Free T4 3024-7

 

Interferences

  • The anticonvulsant drug phenytoin may interfere with total and free T4 levels due to competition for TBG binding sites.
  • Free T4 values may be decreased in patients with non-thyroidal conditions and in patients taking carbamazepine.
  • Thyroid autoantibodies in human serum may interfere and cause falsely elevated free T4 results.