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 < 0.01; IRI: 0.10 ± 0.02 pmol/ml versus 0.05 ± 0.01 pmol/ml; P < 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 < 0.01). Mean IRI levels were significantly higher at all times studied including 180 min (P < 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 < 0.025). After oral glucose, mean C-peptide levels were significantly higher (P < 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 < 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.