International survey of the radioimmunological measurement of serum reverse triiodothyronine

An international survey was conducted to study the performance of the radioimmunoassay of serum reverse triiodothyronine (rT3). Eight laboratories were provided with 10 serum samples containing several concentrations of rT3 and thyroxine (T4). Each laboratory also obtained the same batch of pure rT3. T4 and rT3 were measured by all participants in these samples. Half of the investigators routinely correct their data for T4 cross-reactivity. The results demonstrate that the performance of the rT3 radioimmunoassay is, on the average, satisfactory as judged from recovery experiments. Serum T4 influences the rT3 measurement to a lesser extent in the laboratories where corrections are performed compared with the others. Simple correction methods appear to fail, however, as the effect of a certain T4 concentration was found to vary with the rT3 level in the sample. (Auth.)