The relation of the TRH test to the triiodothyronine (T
3) suppression test was investigated in 43 patients with hyperthyroidism receiving antithyroid drugs for 6 to 27 months. All patients under study were in a euthyroid state, estimated by serum triiodothyronine uptake (T
3U), thyroxine (T
4), triiodothyronine (T
3) and free thyroxine index (FT
4I). The value for 24-hr, uptake after T3 administration was less than 20%in 18 cases, out of which the response to TRH was normal in 15 cases and was absent in 3 cases. On the other hand, out of 25 cases with a 24-hr, uptake value of more than 20%, the response to TRH was absent in 18 cases and was normal in 7 cases. The result of the TRH test correlated well with that from the T
3 suppression test in 33 of 43 cases while in the other 10 cases the responsiveness of thyroid function to TRH was dissociated from its suppressibility with T
3. This finding suggests that the TRH test cannot be a substitute for the T
3 suppression test.
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