[Analytical evaluation of third generation TSH assay. Application to the exploration of thyrotropin suppression in...

Intravenous TRH (200 micrograms Protiréline) injection was performed in 137 patients who had undergone thyroidectomy for differentiated thyroid carcinoma and treated by a suppressive thyroid therapy. Serum thyroid stimulating hormone (TSH) was measured before and 30 minutes after the injection. 143 tests were performed. The TSH method used was a chemiluminometric assay (Chiron Diagnostics). We study the analytical characteristics of the method. The limit of detection was 0.003 mUI/l. Interassay precision showed a functional sensitivity of 0.015 m UI/l with 20% interassay CV in agreement with the third generation criterion. There was a good correlation between TSH values at T0 and T30 minutes. In 143 tests, 40 (28%) showed a basal TSH value less than 0.003 mUI/l; delta TSH (TSH at T30-TSH at T0) was less than 0.015 mUI/l in about 68% of the cases and less than 0.031 mUI/l for the others. In 20% of the tests (29) TSH T0 were between 0.003 and 0.015 mUI/l and delta TSH less than 0.204 mUI/l. Finally the 74 remaining tests with a basal TSH value higher than the functional sensitivity of the method showed delta TSH less than 0.4 mUI/l if the TSH T0 was less than 0.05 mUI/l. The delta TSH remained less than 0.6 mUI/l if the TSH T0 was less than 0.1 mUI/l. Inversely, if TSH T0 was higher than 0.1 mUI/l delta TSH could be higher than 1.0 mUI/l. Third generation TSH assay allows us to abandon TRH test for the follow-up of thyroidectomized patients with suppressive thyroid hormonotherapy in so far as TSH assay is well established. We conclude that a basal TSH less than the functional sensitivity (0.015 mUI/l) can predict a TSH response to TRH stimulation test less than 0.2 mUI/l.