Altered Glucoregulatory Response to Physiological Infusions of Epinephrine and Glucagon in Hyperthyroidism*
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To study the mechanism of altered glucose homeostasis in hyperthyroidism, the effects of a 2-h physiological infusion of epinephrine (0.05 microgram/kg x min) or glucagon (3 ng/kg x min) on glucose kinetics and glucoregulatory hormones were determined in nine normal subjects and five untreated hyperthyroid patients. Under basal conditions, hyperthyroid patients exhibited increased glucose turnover (2.2 +/- 0.09 vs. 1.62 +/- 0.1 mg/kg x min in normals), a modest hyperglycemia, hyperglucagonemia, and normal levels of plasma insulin, cortisol, and GH. In normal subjects, epinephrine induced a sustained increase in plasma glucose (45 mg/dl), reflecting a transient 100% rise in glucose output, and a sustained 28% decrease in glucose clearance. In hyperthyroid patients, the rise in plasma glucose was significantly lower (22 mg/dl) due to a smaller but sustained increase in glucose output (45%) and the lack of a fall in glucose clearance. Plasma insulin rose to a peak 80% higher than baseline in hyperthyroid patients, whereas in normals it initially declined and then rose to levels 50% higher than basal. Plasma glucagon displayed only minor changes in both groups. Glucagon infusion induced similar increments in plasma glucagon levels in the two groups (120-150 pg/ml). Insulin, cortisol, and GH remained unchanged. Plasma glucose rose by 4 mg/dl in hyperthyroid patients and by 11 mg/dl in normal subjects. The net increments were significantly lower in the former group (P < 0.05-0.01). Glucose output increased by 40% in normals and returned to baseline by 75 min, whereas it increased by only 15% in hyperthyroid patients and remained above baseline until the end of the infusion. Glucagon had no appreciable effect on glucose clearance in either group. We conclude that hyperthyroidism is characterized by 1) increased glucose turnover and hyperglucagonemia in the basal state, 2) a reduced glucemic response to physiological infusions of epinephrine and glucagon, 3) a sustained response of glucose production to epinephrine and glucagon, and 4) the lack of epinephrine-induced suppression of glucose clearance, presumably due to an exaggerated response of insulin secretion to epinephrine.
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