Simplified radioimmunoassay for triiodothyronine: evaluation of a new commercial kit.

A new triiodothyronine (T3) radioimmunoassay kit offers simplicity of method, short incubation time, specificity, and reproducibility. 125I-labeled T3 and unknown serum (or standard) are added to alkaline buffered (pH 12.4) Sephadex G-25 columns; the T3 is freed of proteins with an alkaline buffer wash, antiserum is added, and the columns are incubated at room temperature for 2 h. "Free" hormone (i.e., unbound to antiserum) remains on the column after a second wash. The radioactivity on the column is counted and related to the total activity added. The T3 standards produced a straight line on log-logit plot; reactivity of analytical-grade thyroxine was 0.6% on weight basis; and the analytical recovery of T3 added to a T3-free serum was complete. Euthroid, hyperthyroid, and hypothyroid ranges were 0.74-2.64, 2.91-7.52, and 0-1.21 mug/liter, respectively. These values correlated well with both the clinical status of patients and with values obtained by an established radioimmunoassay method. The procedure is simply done; results may be obtained in less than 4 h.