Ability of two new thyrotropin (TSH) assays to separate hyperthyroid patients from euthyroid patients with low TSH

We evaluated the ability of new thyrotropin (TSH) assays to separate hyperthyroid (n = 50) patients from clinically euthyroid subjects with low TSH values (nodular goiter, n = 20, and nonthyroidal illness, n = 22). Only patients whose serum TSH was < 0.1 mIU/L by immunoradiometric assay were included. We used a new immunofluorometric method based on time-resolved fluorescence (TR-IFMA) and a new immunochemiluminometric assay (ICMA) to measure TSH in serum. Although the differences between the hyperthyroid patients and the euthyroid patients differed from each other by both methods (P = 0.0012 for TR-IFMA and P < 0.0001 for ICMA), there was no cutoff point that could definitely separate the groups. Thus, it is not possible to draw any definite conclusions on whether a patient is hyperthyroid or not, solely on the basis of TSH concentration measured with these new TSH assays.