Evaluation of thyrotropin secretion before and after TRH by third 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 clinical significance of subnormal basal TSH (b-TSH) values (< 0.2 microU/ml), we investigated b-TSH and TRH-stimulated TSH (r-TSH) by means of a new, highly sensitive immunochemiluminometric assay in 105 euthyroid subjects, 45 patients with overt hyperthyroidism and 18 patients suspected of having subclinical hyperthyroidism. A weak, albeit statistically significant, correlation (r = 0.48) was found between b-TSH and r-TSH and also between b-TSH and delta-TSH (r = 0.31) in euthyroid subjects. Consideration of b-TSH alone correctly identified 90 % of euthyroid subjects in this group; 10 of 105 apparently euthyroid subjects presented delta-TSH suggesting subclinical hyperthyroidism. While b-TSH was detectable (> 0.04 microU/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 patients with subclinical hyperthyroidism exhibited a blunted TSH response to stimulation (delta-TSH < 2 microU/ml). These results suggest that although the new generation of TSH assays can be a valuable addition to the diagnostic arsenal of thyroid function tests, certain limitations must still be accepted. Specifically, b-TSH in the "grey zone" (0.1-0.2 microU/ml) appears to be a less than reliable predictor of thyroid function.