Positive interference in Radioimmunoassay determination of serum total T3

Determination of thyrotropin, total and free thyroxine and triiodothyronine are widely used as diagnostic methods for thyroid function evaluation. There have been numerous reports of interferences in thyroid hormone immunoassays. Herein, a prominent positive interference is reported for a radioimmunoassay kit of total triiodothyronine. In this study, a total of 3471 patients, referred by endocrinologists, were examined by serum level measurement of thyroid stimulating hormone (TSH), total thyroxine (T4) and triiodothyronine (T3). T3 analysis was made through a competitive solid-phase radio labeled (I125) immunoassay by T3 Izotop kit (Izotop Co. Budapest, Hungary). The presence of T3 assay interference was considered probable if the hormone profile was inconsistent with the clinical picture and/or the obtained value for T3 was extremely deviated from normal levels, i.e. it was above 780 ng/dL. For such patients, the possibility of T3 assay interference was investigated through re-measuring T3 level by another RIA kit (Immunotech kit, Marseille, France). Among 3471 patients studied, 40 cases (36 women, 4 men), with a mean±D age of 38.8±15.0 years, had T3 serum levels inconsistent with their clinical pictures and/or above 780 ng/dL. The mean serum levels of T4 and TSH in this group were 9.0±2.0 μg/dL and 1.79±1.47 μU/mL, respectively. In all these 40 cases, T3 level was in the normal range according to T3-Immunotech kit (with a mean±SD of 132.k1±31.0 ng/dL), in accordance with their clinical pictures. Results clearly indicate the presence of positive interference(s) in some of T3 assays made with Izotop T3-RIA kits. The probable explanations for the observed positive interference are discussed.