Fasting and postabsorptive hepatic glucose and insulin metabolism in hyperthyroidism

The effect of thyroid hormone excess on hepatic glucose balances and fractional hepatic extraction of insulin and glucagon was examined in six conscious dogs with catheters in the portal vein, hepatic vein, and femoral artery and Doppler flow probes on the portal vein and hepatic artery. An oral glucose tolerance test was performed before and after the animals were made hyperthyroid by intramuscular thyroxine administration (100 micrograms.kg-1.day-1) for 10 days. In the basal state and after oral glucose, insulin and glucagon levels in the three vessels and the basal fractional hepatic extraction of insulin and glucagon were not significantly modified by thyroid hormone. These results suggest that in short-term thyrotoxicosis insulin secretion is not impaired, and the rise in fasting plasma glucose and increased hepatic glucose production could reflect hepatic insulin resistance, increased availability of precursors for gluconeogenesis, or increased glycogenolysis. Hyperthyroidism significantly increased basal flows in the portal vein (14.7 +/- 0.6 vs. 12.9 +/- 0.5 ml.kg-1.min-1), the hepatic artery (4.8 +/- 0.3 vs. 3.9 +/- 0.2 ml.kg-1.min-1) and vein (19.6 +/- 0.7 vs. 16.9 +/- 0.4 ml.kg-1.min-1), the fasting plasma glucose concentration (104 +/- 3 vs. 92 +/- 2 mg/dl), and basal hepatic glucose output (2.1 +/- 0.2 vs. 1.5 +/- 0.2 mg.kg-1.min-1). It did not alter the nonhepatic splanchnic uptake of glucose, the percent of orally administered glucose that appeared in the portal vein (47 +/- 2 vs. 45 +/- 11%), the percent of hepatic uptake of glucose (59 +/- 11 vs. 74 +/- 22%), or the shape of the glucose tolerance test.