MEASUREMENT OF CHEMILUMINESCENT THYROTROP IN IN PATIENTS WITH LOW MYOTROPIN AND NORMAL THYROID-HORMONES LEVELS

The performances of a new << 3rd generation >> chemoluminescence TSH a ssay (TSH ICMA) with a functional sensitivity of 0.005 mU/l were compa red with those of an << ultrasensitive >> TSH immunoradiometric assay (TSH IRMA) in a series of patients characterised by a TSH IRMA less th an 0.20 mU/l and normal free thyroxin (T4 L) and triodothyronine (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 un dergoing Tc-99m-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/1, 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, TS H 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 cas es. A satisfactory concordance was observed between the two methods. I n conclusion, the two TSH assays, IRMA and ICMA, provide globally comp arable 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 wi th the frank hyperthyroid zone in patients with non-thyroid disease ar gue in favour of the use of this new assay method.