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.
Martin I. Surks, Alan R. Schadlow, Jack H. Oppenheimer