The relation of the TRHtest to the triiodothyronine (T3) 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 (T3U), thyroxine (T4), triiodothyronine (T3) and free thyroxine index (FT4I). The value for 24-hr, uptake after T3 administration was less than 20%in 18 cases, out of which the response to TRHwas normal in 15 cases and was absent

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... in 3 cases. On the other hand, out of 25 cases with a 24-hr, uptake value of more than 20%, the response to TRHwas absent in 18 cases and was normal in 7 cases. The result of the TRHtest correlated well with that from the T3 suppression test in 33 of 43 cases while in theother 10 cases the responsiveness of thyroid function to TRHwas dissociated from its suppressibility with T3. This finding suggests that the TRHtest cannot be a substitute for the T3 suppression test.