Hyperthyroid Graves' disease causes insulin antagonism.

Oral glucose tolerance and pancreatic suppression tests were carried out in six young hyperthyroid men before treatment and 2-4 weeks after thyroid function tests returned to normal and in six normal men matched for age and weight. Glucose intolerance was present in the hyperthyroid individuals. It improved, but did not return to normal, after treatment. Insulin secretion was increased, and the peak values were delayed in the patients. Treatment did not influence the amount or pattern of the insulin response. Steady state plasma glucose levels (milligrams per dl +/- SEM) were significantly (P less than 0.001) higher in the hyperthyroid patients before treatment (128.6 +/- 2.4) than in the normal subjects (86.1 +/- 3.1). Although after treatment, steady state plasma glucose levels (116.1 +/- 2.7) decreased significantly (P less than 0.001), they remained higher than those in normal subjects (P less than 0.01). Therefore, insulin action was impaired in hyperthyroidism and had not returned to normal by 2-4 weeks after thyroid function tests were in the normal range.