The combined radioimmunoassay of triiodothyronine and thyroxine.

An RIA method for the concurrent determination of serum triiodothyronine (T3) and thyroxine (T4) is described. The extraction of the hormones from serum and competitive binding reactions using specific T3 and T4 antisera were conducted in sequence in a single Sephadex column. Precision studies resulted in average interassay relative standard deviations for T3 and T4 of 17 and 9.5%, respectively. The assay also exhibited satisfactory recovery and correlation with reference procedures (T3: y = 0.84x -- 36, r = 0.98; T4: y = 0.96x + 0.06, r = 0.98, where y is the value obtained by the combined method and x is the value obtained by the reference method.