A New Economical Method for Determination of Triiodothyronine (T₃) Uptake Using Talc Tablets

Abstract

The determination of T₃ uptake is necessary to find the free thyroxine index, increasingly used as the simplest accurate measure of thyroid dysfunction. A T₃ uptake determination requires a reproducible, readily automated, and cost-effective method of adsorbent separation. Five adsorbent separating reagents were studied: hemoglobin-coated charcoal, dextrancoated charcoal, magnesium carbonate, anion-exchange resin, and talc tablets. Of these, binding efficiency was best with talc tablets. Talc binding was insensitive to incubation time, eliminating problems associated with other reagents. To maintain the conventional normal range for T₃ uptake (25-35%), the quantities of talc, tracer, and serum used required adjustment. Results obtained with talc were compared to those obtained with a commercial kit, using paired sera from 897 patients. The correlation coefficient was 0.95 (P<0.0005). The coefficients of variation (CV) for 36 runs were 4.6%, 2.3%, and 2.1% inter-assay, and 3.3 %, 1.7%, and 1.04% intra-assay for hypothyroid, euthyroid, and hyperthyroid controls, respectively.