Enzyme-amplified immunoassays: a new ultrasensitive assay of thyrotropin evaluated.

An enzyme-amplified immunoassay for thyrotropin has been evaluated. The lower limit of detection of the assay (mean + 3 SD of the zero standard) was 0.037 milli-int.unit/L and the precision was good, giving a working range of 0.13-24.0 milli-int.units/L. The accuracy of the assay was good, as judged from analytical-recovery experiments, analysis of external quality-assessment samples and comparison with in-house assays. No significant interferences or cross reactivities were identified. Assay of a 25-microL serum sample on the microtiter plate takes 3 h. Values for healthy subjects ranged from 0.4 to 4.0 milli-int.units/L. For 32 thyrotoxic patients thyrotropin concentrations were clearly suppressed and completely distinct from normal values. Patients taking triiodothyronine also showed lower concentrations. The enzyme-amplified immunoassay may thus be a suitable "front line" test for assessing thyroid function.