COMPARISON OF 2ND AND 3RD GENERATION METHODS FOR MEASUREMENT OF SERUMTHYROTROPIN IN PATIENTS WITH OVERT HYPERTHYROIDISM, PATIENTS RECEIVING THYROXINE THERAPY, AND THOSE WITH NONTHYROIDAL ILLNESS

We compared serum TSH results determined in second and third generatio n assays in patients with thyroid disease and nonthyroidal illnesses ( NTIs) to evaluate the usefulness of the more sensitive assay. We studi ed 19 subjects with untreated hyperthyroidism, 12 hyperthyroid subject s sampled at 4-week intervals after beginning carbimazole, 153 subject s receiving T-4 replacement, and 300 hospital in-patients with a varie ty of NTIs. Serum TSH was measured, using a second generation immunome tric method, together with free T-4 and free T-3. Samples with subnorm al TSH (<0.5 mU/L) were reassayed, using a more sensitive chemilumines cent immunometric method. Both assays revealed undetectable serum TSH levels in 18 of 19 overtly hyperthyroid patients. Undetectable TSH val ues (in both assays) were found in 30 of 33 patients with low serum TS H levels who were receiving treatment for hyperthyroidism, in associat ion with normal thyroid hormone levels in 11. Undetectable TSH was evi dent in both patients receiving T-4 and those with NTI, but use of the more sensitive assay led to a reduction in the number of subjects wit h undetectable TSH compared with the second generation results (T-4-tr eated, 55 vs. 77 cases; NTI, 13 vs. 19 cases). There was a significant correlation between serum TSH and free T-4 in the whole group on T-4 (P < 0.001) and in those receiving T-4 With low TSH (r = -0.33; P < 0. 05); no significant correlation was evident in subjects with low serum TSH levels associated with NTI. An improvement in assay sensitivity l ed to a reduction in the number of patients being treated with T-4 or with NTI in whom serum TSH was undetectable and, hence, an increase in those in whom overt hyperthyroidism could be excluded. Undetectable T SH results, even in a third generation assay, are not diagnostic of ov ert hyperthyroidism, but are also found in subjects with treated thyro id disease and NTI.