Temperature effects on free-thyroxine measurements: analytical and clinical consequences

Area of Science:

• Clinical Chemistry
• Endocrinology
• Immunoassay Development

Background:

• Accurate measurement of free thyroxine (FT4) is essential for diagnosing thyroid disorders.
• Existing FT4 assays, including radioimmunoassays and fluoroimmunoassays, vary in their methodologies.
• Temperature-dependent variations in thyroxine-binding globulin (TBG) affinity can affect FT4 measurements.

Purpose of the Study:

• To compare the analytical and clinical performance of two-step FT4 assays (Spectria, Delfia) against a direct radioimmunoassay (Nichols).
• To investigate the influence of temperature on FT4 assay performance and calibration.
• To determine optimal assay conditions for accurate in vivo FT4 concentration measurement.

Main Methods:

• Analytical and clinical performance evaluation of Spectria, Delfia, and Nichols FT4 assays.
• Testing sera from 135 healthy subjects and 61 patients with and without thyroidal illnesses.
• Investigating the effect of temperature (20°C vs. 37°C) on assay results and TBG binding affinity.

Main Results:

• All three FT4 assays demonstrated comparable analytical performance.
• Significant differences in measured FT4 were observed between assays in euthyroid patients.
• Increased temperature (37°C) significantly decreases TBG-T4 binding affinity, potentially doubling FT4 levels.
• Two-step assays performed at room temperature with non-standardized calibration yield misleading FT4 results, particularly in samples with binding abnormalities.

Conclusions:

• Temperature significantly influences FT4 measurement accuracy due to altered TBG binding.
• Two-step FT4 assays require calibration and performance at body temperature (37°C) for reliable clinical utility.
• Establishing reference values at 37°C is recommended for accurate FT4 assessment across diverse patient populations.