Immunoenzymatic quantification of low concentrations of thyrotropin.

We evaluated an immunoenzymatic assay (Abbott HTSH EIA) for thyrotropin (TSH) as a tool for detecting hyperthyroidism and for monitoring thyroid hormone suppressive therapy in patients with nodular goiter, thyroid carcinoma, and hypopituitarism. We also tested with thyroliberin (TRH), to determine the correlation between peak and basal TSH in suppressed patients. For comparison, we used a nonequilibrium radioimmunoassay optimized for maximum sensitivity (J Clin Endocrinol Metab 1975;41:676). Hyperthyroid patients with values for either or both triiodothyronine and thyroxin above the normal reference interval had Abbott assay values less than or equal to 0.2 milli-int. unit/L, clearly below the Abbott assay normal range, as determined in 116 euthyroid subjects. We detected one-third of the suppressed patients (greater than or equal to 0.3 milli-int. unit/L) with RIA, 69% with the Abbott assay (TSH greater than or equal to 0.04 milli-int. unit/L). Only 20% of patients with undetectable basal TSH values in the Abbott assay responded to TRH with a detectable peak TSH value; the peak TSH value after TRH was proportional to the basal TSH value. A single basal TSH measurement by the Abbott HTSH EIA should be adequate for monitoring the degree of thyroidal suppression in thyroid-hormone-treated patients.