[Proposed reference values for nine methods for measurement of free thyroxin]

Area of Science:

• Clinical Chemistry
• Endocrinology
• Laboratory Medicine

Background:

• Accurate measurement of free thyroxine (FT4) is crucial for diagnosing thyroid disorders.
• Numerous commercial immunoassay kits are available for FT4 determination, but performance can vary.
• Establishing reliable reference ranges is essential for interpreting patient results.

Purpose of the Study:

• To compare the performance of nine different kits for measuring free thyroxine.
• To determine reference values for free thyroxine in an adult population.
• To assess the agreement and discrepancies between various FT4 assay methods.

Main Methods:

• A comparative analytical study was conducted using nine commercial immunoassay kits.
• Reference values were determined in a control group of 81 women and 73 men.
• Correlation analysis was performed to evaluate the agreement between the kits.

Main Results:

• Significant differences in results were observed across the nine evaluated free thyroxine kits.
• The determined reference ranges for FT4 were generally narrower than those provided by the kit manufacturers.
• Inter-kit correlations indicated substantial variability in free thyroxine measurements.

Conclusions:

• The choice of free thyroxine assay kit can significantly impact patient results and diagnostic interpretation.
• Manufacturers' reference ranges may not accurately reflect the performance of their kits in a given population.
• Further standardization or clear reporting of assay-specific reference intervals is warranted for free thyroxine testing.