Sensitive radioimmunoassay for thyrotropin by use of commercially available tracer and antibody

The time course of the association and dissociation of radiolabeled thyrotropin with its antibody was studied. The binding is maximal after five days of incubation. The dissociation is much slower. Highest sensitivity is obtained by a total incubation time of five days with addition of the labeled ligand after three days of preincubation with the unlabeled thyrotropin. An improvement in the sensitivity from 0.4 mU/l to 0.08 mU/l is obtained by that method compared with equilibrium technique thus significantly diminishing the percentage of undetectable serum levels of euthyroid and hyperthyroid subjects. Average sensitivity is 0.14 mU/l, intraassay variance is 2%, interassay variance is about 13% in the normal range. Normal values for euthyroid subjects averaged 1.53 mU/l, for hyperthyroid subjects 0.19 mU/l.