A sensitive and practical immunoradiometric assay of thyrotropin.

We describe a two-site immunoradiometric assay for thyrotropin (TSH) in serum, based on use of two monoclonal antibodies directed against two separate antigenic determinants on the TSH molecule. One antibody is immobilized on polystyrene beads; the other is radioiodinated by a modified Chloramine T method. The detection limit of the assay is 0.02 milli-int. unit/L. The working range (CV less than 10%) is from 0.1 to greater than 50 milli-int. units/L. The log mean concentration of TSH in sera collected from 100 euthyroid subjects between 08:00 and 11:00 hours was 1.9 milli-int. units/L, the range 0.4-5.4 milli-int. units/L. Values for hyperthyroid patients and thyroid-cancer patients being treated with thyroxin were much lower than those for euthyroid persons. Results by this new assay correlated excellently with those by our conventional radioimmunoassay (r = 0.99) and also with a sensitive immunofluorometric TSH method (Delfia TSH) (r = 0.99).