Performance characteristics of seven automated thyroxine and T-uptake methods

Estimates of the free thyroxine concentration can be made using measurements of thyroxine (T4) and the thyroid hormone binding ratio (THBR, usually reported as T-uptake) according to the formula free thyroxine index (FTI)=T4xTHBR. We evaluated the performance characteristics of 7 pairs of automated T4 and T-uptake methods.

We evaluated the Architect i2000, AxSYM, ADVIA Centaur, UniCel DxI 800, Immulite 2000, Modular E170 and Vitros ECi methods for T4 and T-uptake. Imprecision was assessed by duplicate determinations on 3 levels of quality control material per run, 2 runs per day, on 5 separate days. Method comparison studies were performed with 205 patient samples and 68 samples from subjects in the second or third trimester of pregnancy. Results for both imprecision and method comparison were converted to the THBR as outlined by each method's manufacturer.

Overall CVs for T4 and THBR methods were 3.2-8.9% and 1.2-6.7%, respectively. T4 methods generally agreed well. The THBR methods did not agree nearly as well. Comparison of FTI results with free T4 by equilibrium dialysis showed good correlation but different slopes.

All methods show adequate precision but the THBR and FTI results are not well standardized.