Modification of a new commercial time-resolved immunofluorometric assay for thyrotropin (DelfiaR hTSH Ultra) by prediluting samples, reducing tracer concentration, and increasing total reaction volume offered improved functional sensitivity of 0.005 mIU l-1 (20% between-assay CV) over the functional sensitivity limit of 0.014 mIU l-1 of the unmodified TSH assay. Serum TSH concentrations of 50 hyperthyroid patients and 24 clinically euthyroid patients with serum TSH < 0.1 mIU l-1 by a second generation immunoradiometric assay were analysed by both the original and modified TSH-IFMA method. The discrimination was better with the modified method, according to ROC-analysis (p = 0.027). In conclusion, these modifications enhanced functional sensitivity and improved the discriminatory power of the assay in separating hyperthyroid patients from euthyroid subjects with low TSH concentrations.