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Test Code Billings Clinic: 4032 T4 (Thyroxine), Total Only, Plasma or Serum

Performing Laboratory

Billings Clinic Laboratory: Chemistry

Useful For

  • Monitoring treatment with synthetic hormones (synthetic triiodothyronine will cause a low total thyroxine [T4])
  • Monitoring treatment of hyperthyroidism with thiouracil and other antithyroid drugs

  • Total T4 levels offer a good index of thyroid function when the thyroid-binding globulin is normal and nonthyroidal illness is not present

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 Ambient 8 hours
  Refrigerated 2 days
  Frozen (preferred) 1 month

 

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

Males:

0 - 1 day:  8.0 - 20.0 µg/dL

1 - 3 days:  5.0 - 15.0 µg/dL

3 days - 1 month:  6.0 - 14.0 µg/dL

1 month - 1 year:  4.5 - 11.0 µg/dL

1 - 6 years:  4.5 - 10.0 µg/dL

6 - 18 years:  5.5 - 11.0 µg/dL

> 18 years:  4.5 - 12.1 µg/dL

 

Females:

0 - 1 day:  8.0 - 20.0 µg/dL

1 - 3 days:  5.0 - 15.0 µg/dL

3 days - 1 month:  6.0 - 14.0 µg/dL

1 month - 1 year:  4.5 - 11.0 µg/dL

1 - 6 years:  4.5 - 10.0 µg/dL

6 - 18 years:  5.5 - 11.0 µg/dL

> 18 years:  4.8 - 13.9 µg/dL

Day(s) Test Set Up

Daily

Test Classification and CPT Coding

84436 - Thyroxine; total
 

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
Total T4 3026-2

 

Interferences

The anticonvulsant drug phenytoin may interfere with total and free T4 levels due to competition for TBG binding sites.