Blood glucose levels are kept in a narrow range in the body by hormone regulation. Blood glucose levels below 70 mg/dL can cause symptoms of hypoglycemia ranging from shakiness, clamminess, headache, nausea and confusion to coma and death. On the other hand, persistent fasting levels above 100 mg/dL can damage the eyes, kidneys, nerves and cardiovascular system. Hormones that regulate glucose levels in the blood are insulin, glucagon, epinephrine and norepinephrine, cortisol, growth hormone, thyroid hormone, the incretin hormones and amylin.
Insulin
Insulin is the primary hormone that regulates blood glucose levels. Beta-cells in the pancreas produce and secrete insulin in response to increasing levels of glucose in the blood. Insulin targets almost all cells in the body to increase their uptake of glucose, thereby reducing blood glucose levels. In certain tissues such as the liver, insulin promotes the storage of glucose as glycogen so that it may be utilized in times when glucose levels are low. The actions of insulin all work to reduce blood glucose levels. Defects in insulin secretion and/or insulin response profoundly affect blood glucose levels and cause diabetes.
Glucagon
If glucose levels are low in the blood, the alpha-cells in the pancreas produce and secrete glucagon. Glucagon acts on cells in the liver, causing them to break down the stored glycogen back into glucose molecules. A review article in “Diabetes Spectrum” notes that the liver, under normal conditions in the fasted state, is the sole source of glucose. Glucagon stimulates the liver to release stored glucose into the circulation. When the stored glucose reserves are depleted, glucagon stimulates the liver to synthesize glucose from amino acids and fatty acids.
Adrenal Gland Hormones
The "fight-or-flight" hormones, epinephrine and norepinephrine stimulate the liver to break down glycogen and increase glucose synthesis in order to increase the supply of energy to other tissues, such as muscle. Cortisol is a hormone released by the adrenal cortex. It primarily antagonizes insulin actions by decreasing glucose cellular uptake and stimulating glucose synthesis in the liver. When glucose levels drop after exercise or between meals, cortisol and glucagon are released to normalize blood glucose levels.
Growth Hormone and Thyroid Hormone
Growth hormone and thyroid hormone are anabolic hormones involved in the stimulation of cell metabolism and growth. To increase the amount of glucose delivered to cells for growth, they prevent the liver from removing glucose from the blood and storing it as glycogen, and stimulate the liver to synthesize more glucose.
Incretin Hormones
The incretin hormones, GLP-1 and GIP, are released by cells in the gut. These hormones play a role in how fast the contents of a meal are absorbed into the bloodstream and potentiate insulin secretion. Derivatives of GLP-1 have been approved by the Food and Drug Administration to treat hyperglycemia in Type 2 diabetes patients.
Amylin
The review article in Diabetes Spectrum reports that amylin, a second beta-cell derived hormone, was discovered in 1987. Amylin is co-secreted with insulin and complements the actions of insulin to reduce blood glucose levels. It primarily decreases the rate of gastric emptying and blocks glucagon secretion.
