[Thyrotropin assay by chemiluminescence in the diagnosis of dysthyroidism with low thyrotropin and normal thyroid hormones levels].

The performances of a new "3rd generation" chemoluminescence TSH assay (TSH ICMA) with a functional sensitivity of 0.005 mU/l were compared with those of an "ultrasensitive" TSH immunoradiometric assay (TSH IRMA) in a series of patients characterised by a TSH IRMA less than 0.20 mU/l and normal free thyroxin (T4 L) and triiodothyronine (T3 L) concentrations. The 95% cut-off value for hyperthyroidism was 0.03 for TSH ICMA and 0.05 for TSH IRMA. In a first group of 41 subjects undergoing Tc99m thyroid scan, images of multifocal increased uptake or toxic adenoma were associated with a lower TSH ICMA than in patients with a normal isotope scan. TSH ICMA was also lower than TSH IRMA (p < 0.01). At the cut-off value of 0.03 mU/l, the specificity of TSH ICMA was higher than that of TSH IRMA, but the sensitivity were identical. In a second group of 36 patients with severe non-thyroid diseases, TSH ICMA was lower than the cut-off value for hyperthyroidism in 30% of cases, while TSH IRMA was lower than the cut-off value in 40% of cases. A satisfactory concordance was observed between the two methods. In conclusion, the two TSH assays, IRMA and ICMA, provide globally comparable information in subjects with a low TSH and normal T4 L and T3 L. However, the better specificity of TSH ICMA and a smaller overlap with the frank hyperthyroid zone in patients with non-thyroid disease argue in favour of the use of this new assay method.