An improved method for the radioimmunoassay of free-thyroxine in serum dialysates

A convenient, sensitive radioimmunoassay of free thyroxine in serum dialysates is described. The method utilizes a solid phase separation system (pre-formed double antibody) and a relatively short incubation period (220 min) with a staggered addition of tracer. Blanks were low and consistent. The normal range for non-pregnant euthyroid samples (n=59) was 11-23 pmol/l. Third trimester pregnancy samples were mostly within the normal range but at the lower end. Patients on T4 replacement showed a much wider variation in free T4 levels with many samples in the hyperthyroid region. Some hypothyroid samples had undetectable free T4 levels and hyperthyroid samples were frequently greater than 80 pmol/l.