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Background: Abnormal glucose metabolism with impaired glucose tolerance has been documented in patients with thyrotoxicosis, but the pathogenesis is not fully understood. Therefore, the aim of the present study was to study the secretory dysfunction of pancreatic 9-cell and to confirm hyperinsulinemia and hyperproinsulinemia during oral glucose tolerance test(OGTT) in patients with thyrotoxicosis. Methods: After an overnight fast, 75 g OGTT was performed in 10 patients with hyperthyroidism and in 10 healthy control subjects matched for age, sex and hody mass index. Plasma insulin(immuno- reactive insulin, IRI), C-peptide, proinsulin levels were measured by radioimmunoassay. Results: Fasting plasma glucose, insulin and C-peptide levels were similar in the two groups, but plasma proinsulin level was increased in patients with hyperthyroidism(p<0.05). A twofold rise of plasma proinsulin and the proinsulin/insulin ratio was also found in patients with hyperthyroidism during OGTT. The molar ratio of C-peptide and insulin(IRI) was similar in the two groups. Conclusion: Hyperinsulinemia and hyperproinsu- linemia were found in patients with hyperthyroidism compared with controls. Disproportionally increased proinsulin level suggested a pancreatic secretory dysfunction in the patients with hyperthyroidism.