Impaired pancreatic alpha-cell response in hyperthyroidism

Recently, we observed that in hyperthyroid patients, plasma glucagon was not adequately suppressed by an oral glucose load, suggesting altered pancreatic alpha-cell sensitivity. To further assess pancreatic alpha-cell function in hyperthyroidism, plasma glucose, glucagon, and insulin resonses to a protein meal were determined in normal subjects and hyperthyroid patients. Fasting plasma glucose was normal in hyperthyroid patients. A protein meal produced an increase in plasma glucose levels in hyperthyroid patients, whereas in normal subjects protein feeding was followed by a decline in blood glucose levels. Basal glucagon was markedly elevated in three of nine hyperthyroid patients, whereas in the remaining six, fasting plasma glucagon was unaltered. In both groups, protein feeding induced a glucagon rise; however, the increment was significantly smaller in hyperthyroid patients. In hyperthyroidism, fasting plasma insulin was raised and the insulin response to a protein meal was exaggerated. Furthermore, the insulin elevations were sustained and did not return to the basal level by 180 min as observed in normal subjects. We conclude that 1) the plasma glucagon response to a protein meal is blunted in hyperthyroidism, a finding which confirms our recent observation of decreased sensitivity of the pancreatic alpha-cell in hyperthyroidism; 2) fasting hyperinsulinemia with simultaneous euglycemia is consistent with the presence of insulin resistance in hyperthyroidism; and 3) the sustained and exaggerated plasma insulin rise after ingestion of a protein meal suggests hypersensitivity of the pancreatic beta-cell in hyperthyroidism.