The TRG Test. Which is the best index of TSH release?

Prolonged (two hours) TRH tests were performed on 29 controls and 182 thyrotoxicosis patients of varying thyroid status after treatment with radioiodine. The object was to find which of the simple measurements of TSH from the TRH test most faithfully reflected the total amount of TSH (TTSH) released in response to the TRH. The simple indices compared were the absolute levels of TSH (ATSH) achieved at intervals during the test, and the increments in TSH (delta TSH) recorded at the same points in time. TTSH was measured by planimetry of the area beneath the response curve. Patients were classified as normo-responders, hyper-responders or hypo-responders according to normal limits for TTSH deduced from the controls. When each simple index of TSH response was substituted in turn for TTSH, the overall frequency of misclassification was considerably greater for ATSH than for delta TSH. This was largely due to the wide variability of passive or basal TSH secretion, which seriously confused the distinction between normally-responsive and hypo-responsive patients when ATSH was used. The delta TSH measured at 30 minutes gave the best overall results in terms of least classification error and closest correlation with TTSH.