EVALUATION OF THYROTROPIN SECRETION BEFORE AND AFTER TRH BY 3RD-GENERATION CHEMILUMINESCENT ASSAY - ASSESSMENT OF SUBCLINICAL HYPERTHYROIDISM

The recent introduction of third generation assays for TSH has led to a considerable improvement of assay sensitivity. To assess the clinica l significance of subnormal basal TSH (b-TSH) values (< 0.2 muU/ml), w e investigated b-TSH and TRH-stimulated TSH (r-TSH) by means of a new, highly sensitive immunochemiluminometric assay in 105 euthyroid subje cts, 45 patients with overt hyperthyroidism and 18 patients suspected of having subclinical hyperthyroidism. A weak, albeit statistically si gnificant, correlation (r = 0.48) was found between b-TSH and r-TSH an d also between b-TSH and delta-TSH (r = 0.31) in euthyroid subjects. C onsideration of b-TSH alone correctly identified 90 % of euthyroid sub jects in this group; 10 of 105 apparently euthyroid subjects presented delta-TSH suggesting subclinical hyperthyroidism. While b-TSH was det ectable (>0.04 muU/ml) in 8 of 45 (18 %) of hyperthyroid patients, all (100 %) were abnormal in both b-TSH and r-TSH. 14 of 18 (78 %) of pat ients with subclinical hyperthyroidism exhibited a blunted TSH respons e to stimulation (delta-TSH <2 muU/ml). These results suggest that alt hough the new generation of TSH assays can be a valuable addition to t he diagnostic arsenal of thyroid function tests, certain limitations m ust still be accepted. Specifically, b-TSH in the ''grey zone'' (0.1-0 .2 muU/ml) appears to be a less than reliable predictor of thyroid fun ction.