Oral and intravenous glucose-induced insulin secretion in hyperthyroid patients

To elucidate glucose intolerance in hyperthyroidism, insulin response to oral (75 g) and intravenous (IV) (20 g) glucose administration was investigated in 18 hyperthyroid patients and six normal control subjects. In oral glucose tolerance tests (OGTT), plasma insulin and C-peptide levels in hyperthyroid patients were not significantly different from that in controls; however, an impaired blood sugar response was observed in hyperthyroid patients. In IVGTT, blood sugar, plasma insulin, and C-peptide levels were significantly higher in hyperthyroid patients than in controls. Insulin secretion in proportion to blood sugar stimulus (the sum of increment in insulin divided by the sum of increment in blood sugar after glucose load, sigma delta IRI/sigma delta BS) in IVGTT was similar in hyperthyroid patients and controls; however, that in OGTT was significantly lower in hyperthyroid patients. After thyroid function tests had returned to normal by treatment with thiamazole, glucose tolerance and sigma delta IRI/sigma delta BS in OGTT were almost normalized. These results indicate that decreased insulin secretion after oral glucose may have an important role in abnormal oral glucose metabolism in hyperthyroidism.