Serum TSH levels were ,measured by radioimmunoassay before and afeer intravenous administration of synthetic thyrotropin-releasing hormone(TRH) to 15 normal subjects and 55 patients with primary thyroid disease(l4 patients with euthyroidism, 24 patients with thyrotoxicosis and 17 patients with hypothyroidism) to evaluate pituitary TSH reserve and its diagnostic availability. The observed results were as follows. 1) In normal subjects, serum TSH responses to synthetic TRH were 3.2±1.0 at 0 min(baseline TSH level), 8.0±4.0 at I0 min, 11.7±5.0 at 20min, 13.7±7.1 at 80min, 9.7±5.0 at 60min., 5.2±2.0 at 120min. and 3.6±0.4 μU/ml at 180 min. Serum TSH peaked at 20∼30 minutes and returned nearly to baseline at 180 minutes. 2) In euthyroid group, serum TSH responses to synthetic TRH were 3.3±1.6 at 0 min, 8.6±8.0 at l0 min, 10.9±8. 5 at 20 min, 12.5±8.4 at 30 min, 9.0±5.9 at 60 min, 5.6±2.6 at 120 min and 3.5±1.3 μU/ml at 180 min. No significant difference revealed between euthyroid group and normal subjects(p>0.05). 3) In hyperthyroid group, serum TSH responses to synttretic TRH were 1.5±0.6 at 0 min, 2.2±0.8 at I0 min., 2.3±1.0 at 20 min., 2.4±1.5 at 30 min., 2.1±1.1 at 60 min,, 1.9±0.2 at 120 min, and 1. 5±0.8 μU/ml, at 180 min., No response to TRH showed. 4) In hypothyroid group, mean values of serum TSH response to synthetic TRH were 42.0 at 0 min., 60.6 at 10 min., 124.8 at 20 min., 123.0 at 30 min, 101.6 at 60 min., 64.3 at 120 min. and 15.5 μU/ml at 180 min., Patients with primary hypothyroidism showed an exaggerated TSH response to synthetic TRH despite their high basal TSH. 5) Side effects attending synthetic TRH administration were transient nausea(59.0%), desire to micturate(59.0%), feeling of flushing(l9.7%), dizziness (45.9%), metallic taste(9.8%) and headache(l9.7%). Any side effect didn't show in 16.4%. These symptoms began almost immediately after TRH intravenous injection and lasted several minutes, and not related to dose or response in the person experiencing it. It can be concluded that TRH stimulation test would be useful as a means of detecting early or marginal forms of hypothyroidism and hyperthyroidism in the absence of abnormalities in conventional indices of thyroid function.