Further observations on radioimmunoassay for thyroglobulin-binding capacity in human serum

A simplified radioimmunoassay for measuring thyroglobulin-binding capacity in human serum is described. Thyroglobulin (Tg) is labelled with lactoperoxidase (specific activity 5 microCi/microgram) which makes it possible to measure a binding capacity of more than 10 ng/ml of serum. The standard curve is plotted measuring the percentage of bound 125I-Tg against the dilution of 1 mg/ml Tg capacity from a pool of Tg antibodies from different individual serum samples. When polyethyleneglycol and a second antibody were compared in their precipitation of the immune complexes, the latter provided a better precipitation. The stability of the labelled Tg stored at --20 degrees C was checked at different intervals of time by ultracentrifugation and immunoreactivity. It was found that the labelled Tg can be used up to one month after iodination. Fragments produced during storage at --20 degrees C only partially reacted with Tg antibodies. Screening of 300 serum samples was performed by haemagglutination and binding capacity; of 45 positive serum samples, 28 were positive only by binding capacity, thus demonstrating that this method is far more sensitive, accurate and precise than those currently employed.