Determination of triiodothyronine in serum by enzyme- and radioimmunoassay: a comparative study

An evaluation of a heterogeneous enzyme immunoassay for determination of triiodothyronine in serum (Enzymun-Test T3, Boehringer Mannheim) is presented. The enzyme immunoassay was compared with the laboratory routine radioimmunoassay. The precision of both assays was satisfactory at triiodothyronine concentrations between 1.0 and 8.0 nmol/l (coefficients of variation from day to day less than or equal to 10%). Average recoveries of triiodothyronine in pool sera ranged with the Enzymun-Test T3 from 96--104% and with the radioimmunoassay from 88--111%. A comparison of the results obtained by Enzymun-Test T3 and the radioimmunoassay in a series of 103 patients showed a good correlation between both methods. L-thyroxine did not cause a relevant cross-reaction in the enzyme immunoassay. About 20 unknown samples can be analyzed in triplicate by Enzymun-Test T3 within 260 minutes.