A New Radioimmunoassay for Plasma l-Triiodothyronine: Measurements in Thyroid Disease and in Patients Maintained on Hormonal Replacement

Abstract

A new procedure for the radioimmunoassay of l-triiodothyronine (T₃) in human plasma is described in which the iodothyronines are separated from the plasma proteins before incubation with a specific antiserum to T₃. The antibody bound and free T₃ are separated with dextran-coated charcoal. In this system, the mean recovery of T₃ added to plasma was 97.9% and both in vitro conversion of l-thyroxine (T₄) to T₃ and cross-reaction between T₄ and the anti-T₃ antibody were undetectable (less than 0.1%). The assay procedure allowed the measurement of T₃ in up to 0.5 ml of plasma resulting in improved assay sensitivity (6 ng/100 ml). The mean plasma T₃ in normal subjects was 146±24 ng/100 ml (sd). Mean T₃ concentration was increased in hyperthyroidism (665±289 ng/100 ml) and decreased in hypothyroidism (44±26 ng/100 ml). In patients with severe hypothyroidism, plasma T₃ was between 7 and 30 ng/100 ml. Plasma T₃ concentration was relatively constant throughout the day in three euthyroid subjects. In contrast, in hypothyroid subjects on replacement therapy with T₃, a T₄: T₃ combination or desiccated thyroid plasma T₃ was markedly elevated for several hours after ingestion of the medication. Plasma T₃ was unchanged throughout the day in patients treated with T₄. Thus, insofar as plasma T₃ levels are concerned, replacement therapy with T₄ appears to mimic the euthyroid state more closely than other preparations.