Insulin Secretion and Sensitivity in Hyperthyroidism

Summary

To examine the effect of hyperthyroidism on carbohydrate metabolism, we studied glucose-stimulated insulin secretion and glucose utilization in 8 subjects with Graves' disease before and after treatment for hyperthyroidism and 8 age-, sex- and weight-matched normal subjects. Subjects with Graves' disease had significant elevated serum levels of thyroxine (24.81 ± 2.44 ug/dl, mean ± SEM) and triiodothyronine (459 ± 5.5 ng/dl, mean ± SEM). Simultaneous measurement of plasma glucose, serum insulin and C-peptide levels during fasting and every 30 minutes up to 180 minutes after 75 g oral glucose loading was determined. In addition, plasma glucose, serum insulin and serum C-peptide were measured during euglycemic glucose clamp with insulin infusion of 40 mU/m² min^-1.

Mean fasting plasma glucose (P < 0.05, serum insulin (P < 0.005) and serum C-peptide (P < 0.005) levels were significantly higher in the hyperthyroid patients. After glucose loading, the plasma glucose (P < 0.05), serum insulin (P < 0.05) and C-peptide (P < 0.05) responses were significantly higher in hyperthyroid patients at all times up to 180 minutes.

During euglycemic clamp studies, the steady-state serum insulin levels were identical in the two groups. The glucose disposal rate was lower in hyperthyroid patients before treatment (P < 0.01) than in normal subjects. After thyroid function had been normalized for 2 to 4 weeks, the glucose disposal rate increased significantly (P < 0.05), but was still significantly lower than those of normal subjects (P < 0.05).

Our data show that patients with Graves' hyperthyroidism manifest glucose intolerance, hyperinsulinemia and insulin resistance.

Key words

Insulin Secretion - Insulin Sensitivity - Hyperthyroidism