Glucose is a principal fuel metabolized to produce adenosine triphosphate for use in cellular-energy–requiring processes and is vital for normal cell function. Homeostatic mechanisms maintain blood glucose levels within narrow physiologic limits (ie, euglycemia).1 Glucose ranges in dogs and cats vary slightly but generally measure ≈90 mg/dL.2
Glucose homeostasis is a balance between glucose appearing in and disappearing from the blood (Figure). Normoglycemia is maintained by the complex interactions of a group of hormones that exert hyperglycemic or hypoglycemic actions by altering the metabolic pathways that produce or consume glucose.3 Insulin, produced by β cells in the pancreatic islets, is the most important hormone for maintaining glucose homeostasis. Insulin secretion is precisely regulated by glucose. In circulation, it exerts potent hypoglycemic actions by promoting cellular uptake of glucose, stimulating hepatic glycogenesis, and suppressing hepatic gluconeogenesis.4 Several hormones that promote hyperglycemia oppose insulin’s hypoglycemic actions. Glucagon, also of pancreatic islet origin, activates hepatic glycogenolysis and gluconeogenesis pathways that increase net glucose production by the liver.3 Thyroid hormones exert a hyperglycemic action, especially when secreted in excess, as in hyperthyroidism.3 Adrenal catecholamines (eg, epinephrine, norepinephrine),5 cortisol,6 and growth hormone7 also antagonize insulin action. Glucagon, growth hormone, catecholamines, and cortisol are collectively called “counterregulatory hormones” to reflect their functions as insulin antagonists. These hormones are the physiologic foundation of hyperglycemia that develops as part of the “fight-or-flight” response, but individual hormones play roles in various disorders that have insulin resistance as a common pathology.8