TRH stimulation tests and T3 suppression tests were done in 13 patients with clinically suspected mild or early hyperthyroidism who were all conventional thyroid function tests gave results within the accepted normal range. The results were as follows 1) 6 paatients with normal T3 suppression test revealed normal TRH stimulation test and could be easily diagnosed as euthyroidism 2) 7 patients with abnormal T3 suppression test exhibited rao TSH response to TRH stimulation test and could be easily diagnosed as hyperthyroidism. The TRH stimulation test is a single, sensitive and reliable test of thyroid function and can well replace T3 suppression test in the diagnosis of marginal hyperthyroidism.