Thyroxine

Test Code

Alias/See Also

Total T4, Total Thyroxine

CPT Codes
84436

Preferred Specimen

Serum (gold top tube)

Minimum Volume

0.5 mL

Other Acceptable Specimens

Lithium Heparin Plasma (green top tube)

Transport Container

Serum (gold top) or Lithium Heparin Plasma (green top) Tube

Transport Temperature

Room Temperature or Refrigeration

Specimen Stability

Room Temperature - 1 day; Refrigerated - 5 days

Reject Criteria (Eg, hemolysis? Lipemia? Thaw/Other?)

Turbidity, Quantity Not Sufficient, IV contamination

Methodology
Chemiluminescence

Setup Schedule

Daily upon receipt

Report Available

Upon completion of analysis

Limitations

Thyroid hormone autoantibodies in samples may cause interference with this test. Results that are inconsistent with clinical observations indicate the need for additional testing.

Reference Range

5.53 - 11.0 ug/dL

Clinical Significance

Thyroxine accounts for at least 90% of circulating protein-bound iodine. While >99.9% of T4 is protein-bound, primarily to thyroxine-binding globulin (TBG), it is the free fraction that is biologically active. In most patients the total T4 level is a good indicator of thyroid status, however it can sometimes be inadequate, and diagnostic efficiency may be improved by use of a total T4 test in conjunction with other tests. T4 levels may be altered in conditions affecting the capacity of the thyroid hormone binding proteins, e.g. pregnancy, when a test for triiodothyronine (T3) or T4 uptake may be used with the total T4 result to calculate the free T4 index (FT4I) to estimate the level of metabolically active free T4.

The CPT Codes provided in this document are based on AMA guidelines and are for informational purposes only. CPT coding is the sole responsibility of the billing party. Please direct any questions regarding coding to the payor being billed. Any Profile/panel component may be ordered separately. Reflex tests are performed at an additional charge.