Decreased serum C-peptide/insulin molar ratios after oral glucose ingestion in hyperthyroid patients

Since C-peptide/immunoreactive insulin (IRI) molar ratios may reflect hepatic extraction of insulin, we measured simultaneous serum glucose, IRI, and C-peptide levels during fasting and 30, 60, 90, 120, and 180 min after 75 g of oral glucose in 10 hyperthyroid patients and 10 age- and weight-matched controls. Mean fasting serum glucose and IRI levels were significantly higher in the hyperthyroid versus control subjects (glucose: 4.9 +/- 0.3 mmol/L versus 4.36 +/- 0.11 mmol/L, P less than 0.01; IRI: 0.10 +/- 0.02 pmol/ml versus 0.05 +/- 0.01 pmol/ml; P less than 0.025). After glucose, mean serum glucose levels were significantly higher in the hyperthyroid versus control subjects at all times studied except for 180 min (P less than 0.01). Mean IRI levels were significantly higher at all times studied including 180 min (P less than 0.01). Mean fasting C-peptide levels were significantly greater in the hyperthyroid patients compared with the controls (1.2 +/- 0.25 pmol/ml versus 0.62 +/- 0.09 pmol/ml; P less than 0.025). After oral glucose, mean C-peptide levels were significantly higher (P less than 0.025) in the hyperthyroid compared with control subjects at 30-60 min but not at 90-180 min. Molar ratios of C-peptide/IRI were significantly lower (P less than 0.05) in the hyperthyroid versus control subjects at all times studied except fasting. In summary, glucose intolerance and hyperinsulinism occur in hyperthyroidism. In addition, C-peptide/IRI molar ratios are reduced after oral glucose ingestion.