Free Thyroxine (FT4) and Free Triiodothyronine (FT3) Estimate Tests

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Free Hormone Measurement - the Future

The era of immunoassay methods for the quantification of thyroid and steroid hormones in biological fluids began in the 1970's. That era is now drawing to a close. The era of advanced mass spectrometry for quantifying hormones in biological fluids is now emerging.[138] There is no reason to doubt that mass spectrometry will provide better quantification because of greater analytical specificity and less analytical interference than immunoassays. So far such techniques have only being applied to TT4 determination.[139] However, for total hormone assays, the requirement for complete hormone release from protein-hormone complexes will remain. For free hormone assays, the requirement for a physical separation of free hormone and protein bound hormone, prior to quantification will also remain. In order to accomplish the later, new separation technology will be needed, before any method can be regarded as a gold standard. The implicit dilution of small molecules is a limitation of equilibrium dialysis that needs to be overcome. Ultrafiltration shows promise, but current methods are either too leaky or too impractical for this task. Mass spectrometry measurements of hormones that form complexes with serum proteins will only be as good as the specimen preparation steps associated with the quantification. However, the ideal free hormone reference method would be a technique that employs ultrafiltration at 37°C, to avoid dilution effects and the direct measurement of free hormone in the ultrafiltrate by mass spectrometry.

Guideline 17. For Laboratories Performing FT4 and FT3 testing

  • Physicians should have ready access to information on the effects of drugs and the diagnostic accuracy of the test used for assessing the thyroid status of patients with various binding protein abnormalities and severe illnesses.

  • When requested by the physician, the laboratory should be prepared to confirm a questionable result by performing a total hormone measurement or by re-measuring FT4 by a reference method that physically separates free from bound hormone, such as direct equilibrium dialysis or ultrafiltration.

  • Questionable results on specimens should be checked for interference by re-measurement made with a different manufacturer's method. (Send out to a different laboratory if necessary.)