T4 Free (FT4)

Casandra

Casandra Test Code QD74106Version 1 (DRAFT)

Performing Lab

Lab Quest DiagnosticsLab Test ID 866CPT Code 84439

Also known as: FT4

Clinical Use

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Use

Free T4 (FT4) is used to help diagnose hypothyroidism and hyperthyroidism as part of thyroid function testing. The Quest Diagnostics test code 866 uses an analog-based competitive immunoassay performed on the Siemens Centaur automated platform. Although convenient, results may be impacted by abnormal levels of thyroid binding globulin (such as during pregnancy or with estrogen excess), low albumin levels, or dysalbuminemic conditions; results may be biased low or high under such conditions. Repeat testing using equilibrium dialysis may be considered when results are inconsistent with the clinical picture. This method generally has a shorter turnaround time than equilibrium dialysis.

Special Instructions

This immunoassay assumes normal binding protein capacity. In cases of abnormal albumin or TBG (e.g., pregnancy, genetic conditions), results may be biased. Heterophilic antibodies may also interfere. If results seem inconsistent clinically, consider repeat testing by direct equilibrium dialysis.

Limitations

Analog-based immunoassays perform poorly at extremes of hormone-binding capacity. Excess TBG may produce positive bias; TBG deficiency or low albumin may produce negative bias. Familiar dysalbuminemic hyperthyroxinemia and circulating thyroid hormone autoantibodies may also alter results. Heterophilic antibodies can cause erroneous results. Immunoassay results are typically 40–60% lower than equilibrium dialysis, and should be repeated by dialysis if inconsistent with clinical findings.

Test Details