[Cooperative survey for the determinations of L-triiodothyronine (T3) and L-thyroxine (T4): critical evaluation of methods employed (author's transl)].

A cooperative survey of the radioimmunoassays for triiodothyronine was performed among 30 participating laboratories from the Thyroid Association of the German Society for Endocrinology. A total of 16 serum samples were sent frozen to each laboratory for the determination of both triiodothyronine (T3) and thyroxine (T4). It was possible to calculate the recovery rates of the participants from the values determined for an internal standard curve, which was hidden within the 16 samples and was made up by addition of known amounts of T3 and T4, respectively, to an untreated serum low in T3 and T4 (recovery curve). In general, most participants overestimated T3 levels, whereas T4 was determined falsely low. The intraassay coefficient of variation (CV) was determined from triplicate readings of three identical sera included within tor T3, and 4.5 and 30% for T4, respectively. There was no significant cross reaction for T4 in the T3 assays. However, most T3 assays were significantly influenced by variations of the thyroxine binding globulin (TBG) levels between different samples, that were tentatively included. It is suggested, that an improvement of the accuracy of the T3 radioimmunoassays necessitates either prior extraction of T3 or antibodies of higher avidity.