Effect of synthetic TRH on pituitary TSH secretion in man with special reference to the evaluation for TRH test. Synthetic TSH has been administrated to 14 volunteers of euthyroid and various endocrine disorders in dose 500 μg. TRH was assessed by serial measurement of plasma TSH using radioimmunoassay with double antibody technique, serum T4 and T3. Arise in plasma TSH was observed within five minutes after intravenous administration of TRH, values of TSH reached maximum in 15 to 30 minutes in euthyroid subjects. Graded responses were obtained as the dose levels of TRH were increased. In the cases with hypothalamo-pituitary disorder including pituitary adenoma and craniopharyngioma, the TSH response to TRH in patients who showed thyroidal hypofunction was generally reduced. However, there could be observed optimal TRH induced TSH increase in hypothalamo-pituitary disorder having a euthyroid function. Almost all patients with hyperthyroidism showed no response, the plasma TSH level remaining undetectable throughout the studies, some showed a suboptimal response and a few delayed response: the plasma TSH increases only at 60 minutes. In hyperthyroidism there was no correlation between the levels of serum T4 and responseness to TRH. Primary hypothyroid patients have elevated plasma TSH levels at zero time and showed exaggerated response and the peak of TSH increase was somewhat delayed and continued. Longterm glucocorticoid treated patients and two cases of with Cushing's syndrome showed an inhibition of TSH response to TRH.