Diabetes is the most common cause of high glucose levels, says Dr. Melissa Conrad Stöppler. In non-diabetics, high glucose levels are also caused by pancreatitis, overactive thyroid gland, hormone-secreting tumors, pancreatic cancer and severe physical stress.
The medical term for a high glucose level is hyperglycemia, notes WebMD. In diabetics, there are two different types of hyperglycemia: postprandial and fasting. Postprandial hyperglycemia is when a diabetic has a blood glucose level greater than 180 milligrams per deciliter one to two hours after a meal. Fasting hyperglycemia is when a diabetic has a glucose level greater than 130 milligrams per deciliter after fasting for a minimum of eight hours. The causes of hyperglycemia in diabetics include eating too many carbohydrates, decreased activity, illness and skipping medications used to control blood glucose levels.
In non-diabetics, taking certain medications can lead to increased glucose levels, says Dr. Stöppler. Medications that increase glucose levels include beta-blockers, estrogen, prednisone and oral contraceptives. Some tumors secrete excess hormones, resulting in elevated blood glucose levels. Pheochromocytomas, glucagonomas and tumors that secret growth hormone are all examples of tumors that can cause increased blood sugar levels. An endocrine disorder called Cushing's syndrome also affects glucose levels, due to an increased level of cortisol in the blood.