A simple and rapid radioimmunoassay of triiodothyronine in unextracted serum.

A simple, accurate, and rapid procedure for radioimmunoassay (RIA) of triiodothyronine (T3) is described. Serum is denatured by sodium trichloroacetate and the T3 released is allowed to react with 125I-T3-labeled T3 anti-serum. The displaced 125I-T3 is rapidly taken up by an anionic-resin sponge, and this uptake is linearly related to serum T3 concentration. The entire assay procedure was completed in the same tube in about 1 hr. The T3 antibody was specific, and T4-to-T3 conversion in the assay did not occur. Recovery of exogenously added T3 was virtually complete. The mean T3 concentration in 92 euthyroid subjects (142 ng/100 ml) is comparable to the values described in a number of radioimmunoassays for T3.