Correlation of Plasma Free Thyroxine Levels with Insulin Sensitivity and Metabolic Clearance Rate of Insulin in Patients with Hyperthyroid Graves' Disease.

Insulin sensitivity, metabolic clearance rate of insulin (MCR-I) and basal posthepatic insulin delivery rate (BIDR) were investigated by means of euglycemic clamp technique in 8 normal subjects and 8 patients with hyperthyroid Graves' disease. The mean (+/- SD) steady-state glucose infusion rate (SSGIR) was lower in hyperthyroid Graves' patients than in normal subjects (228.9 +/- 57.0 vs. 290.9 +/- 49.4 mg/m2/min, p < 0.05). Both MCR-I and BIDR were higher in hyperthyroid Graves' patients than in normal subjects (1162.9 +/- 517.1 vs. 463.5 +/- 103.9 ml/m2/min, p < 0.005; 17.7 +/- 12.6 vs. 3.6 +/- 0.9 mU/m2/min, p < 0.01, respectively). Plasma free T4 levels showed a close correlation with MCR-I (r = 0.77, p < 0.05) and BIDR (r = 0.81, p < 0.05), respectively, in Graves' patients. These findings indicate that hyperthyroidism is characterized by not only a decrease in insulin sensitivity, but also an increase in basal insulin secretion and the metabolic clearance rate of insulin, which are correlated with plasma free T4 levels.