Insulin-Stimulated Rates of Glucose Uptake in Muscle in Hyperthyroidism: The Importance of Blood Flow

Abstract

In hyperthyroidism, although hepatic insulin resistance is well established, information on the effects of insulin on glucose uptake in skeletal muscle is variable. To investigate this, a meal was given to nine hyperthyroid (HR) and seven euthyroid (EU) subjects. Blood was withdrawn for 360 min from a forearm deep vein and the radial artery for measurements of insulin and glucose. Forearm blood flow (BF) was measured with strain-gauge plethysmography. Glucose flux was calculated as arteriovenous difference multiplied by BF and fractional glucose extraction as arteriovenous difference divided by arterial glucose concentrations. Both groups displayed comparable postprandial glucose levels, with the HR having higher insulin levels than the EU. In the forearm of HR vs. EU: 1) glucose flux was similar [area under the curve (AUC)(0-360) 673 +/- 143 vs. 826 +/- 157 micromol per 100 ml tissue]; 2) BF was increased (AUC(0-360) 3076 +/- 338 vs. 1745 +/- 145 ml per 100 ml tissue, P = 0.005); and 3) fractional glucose extraction was decreased (AUC(0-360) 14.5 +/- 3 vs. 32 +/- 5%min, P = 0.03). These results suggest that, in hyperthyroidism, insulin-stimulated glucose uptake in muscle is impaired; this defect is corrected, at least in part, by the increases in BF.