Serum TSH measurements by a sensitive enzyme immunoassay discriminate euthyroid from hyperthyroid subjects and avoid the need for TRH test during suppressive therapy with L-thyroxine

Abstract

Serum TSH was determined photometrically by a recently developed enzyme immunoassay (EIA) based on the use of a monoclonal antihuman TSH-beta antibody and a polyclonal antiTSH antibody coupled to horseradish peroxidase. The results obtained in patients with various thyroid disorders and in normal controls were compared with those achieved by conventional double antibody radioimmunoassay (RIA). In normal subjects, serum TSH was detectable in all cases by EIA (values ranging from 0.27 to 5.1 mU/L), but only in 76% by RIA. Ninety-two percent of hyperthyroids had undetectable serum TSH by EIA and the remaining 8% had values between 0.2 and 0.4 mU/L. In clinically euthyroid patients with nontoxic goiter, 9% had undetectable serum TSH by EIA, suggesting the presence of autonomously functioning areas within the thyroid. Serum TSH under basal conditions and after TRH stimulation was measured in 45 patients on L-thyroxine suppressive therapy. Undetectable basal serum TSH by EIA was associated with a lack of TSH response to TRH in 95% of cases. Conversely, 37.5% of patients with undetectable basal serum TSH by RIA had a normal or blunted response to TRH. Detectable basal values were predictive of a normal response to TRH by both methods. These data indicate that basal serum TSH measurement by EIA allows an almost complete differentiation of normal from thyrotoxic patients and can avoid the need of the TRH stimulation test.