Concentrations of triiodo-L-thyronine in the plasma and tissues of normal rats, as determined by radioimmunoassay: comparison with results obtained by an isotopic equilibrium technique.

The concentrations of T3 (3',3,5-triiodo-L-thyronine) have been determined by RIA in plasma, liver, kidney, heart, muscle, brain, and lungs of normal male rats. The method involves homogenization, addition of a high specific activity labeled T3 tracer, extraction with ethanol, separation by paper chromatography, elution of the labeled T3 spot, evaporation of an aliquot, and assay of its T3 content by a highly sensitive RIA. In order to validate the results thus obtained, they were compared with the T3 concentrations determined for the same samples using an isotopic equilibrium technique. The data obtained by RIA were in close agreement with those derived from the isotopic equilibrium technique, whenever the latter could be applied accurately. The present RIA method permits the determination of T3 in tissues without imposing the well known limitations of the isotopic equilibrium technique. The concentration of T3 was higher in the kidney than in the liver. In kidney, liver, brain, and lung, it was higher than the concentration in plasma. The concentration of T3 in heart was the same as that in plasma. Similarities and differences with respect to data reported by others are discussed.