Diabetes mellitus is not a single disorder but a heterogeneous group of disorders. All forms are characterized by hyperglycemia and disturbances of carbohydrate, fat, and protein metabolism which are associated with absolute or relative deficiencies of insulin action and/or insulin secretion. The World Health Organization (WHO) developed a now widely accepted classification of the disorder, largely based on clinical characteristics (see Table 1, WHO, 1985).
Jump up ^ Cheng J, Zhang W, Zhang X, Han F, Li X, He X, Li Q, Chen J (May 2014). "Effect of angiotensin-converting enzyme inhibitors and angiotensin II receptor blockers on all-cause mortality, cardiovascular deaths, and cardiovascular events in patients with diabetes mellitus: a meta-analysis". JAMA Internal Medicine. 174 (5): 773–85. doi:10.1001/jamainternmed.2014.348. PMID 24687000.
Excessive thirst typically goes hand-in-hand with increased urination. As your body pulls water out of the tissues to dilute your blood and to rid your body of sugar through the urine, the urge to drink increases. Many people describe this thirst as an unquenchable one. To stay hydrated, you drink excessive amounts of liquids. And if those liquids contain simple sugars (soda, sweet iced tea, lemonade, or juice, for example) your sugars will skyrocket even higher.
Viral infections may be the most important environmental factor in the development of type 1 diabetes mellitus,  probably by initiating or modifying an autoimmune process. Instances have been reported of a direct toxic effect of infection in congenital rubella. One survey suggests enteroviral infection during pregnancy carries an increased risk of type 1 diabetes mellitus in the offspring. Paradoxically, type 1 diabetes mellitus incidence is higher in areas where the overall burden of infectious disease is lower.
Diabetes insipidus is characterized by excessive urination and thirst, as well as a general feeling of weakness. While these can also be symptoms of diabetes mellitus, if you have diabetes insipidus your blood sugar levels will be normal and no sugar present in your urine. Diabetes insipidus is a problem of fluid balance caused by a problem with the kidneys, where they can't stop the excretion of water. Polyuria (excessive urine) and polydipsia (excessive thirst) occur in diabetes mellitus as a reaction to high blood sugar.
People with type 1 diabetes sometimes receive transplantation of an entire pancreas or of only the insulin-producing cells from a donor pancreas. This procedure may allow people with type 1 diabetes mellitus to maintain normal glucose levels. However, because immunosuppressant drugs must be given to prevent the body from rejecting the transplanted cells, pancreas transplantation is usually done only in people who have serious complications due to diabetes or who are receiving another transplanted organ (such as a kidney) and will require immunosuppressant drugs anyway.
Most cases (95%) of type 1 diabetes mellitus are the result of environmental factors interacting with a genetically susceptible person. This interaction leads to the development of autoimmune disease directed at the insulin-producing cells of the pancreatic islets of Langerhans. These cells are progressively destroyed, with insulin deficiency usually developing after the destruction of 90% of islet cells.
According to the Mayo Clinic, doctors may use other tests to diagnose diabetes. For example, they may conduct a fasting blood glucose test, which is a blood glucose test done after a night of fasting. While a fasting blood sugar level of less than 100 milligrams per deciliter (mg/dL) is normal, one that is between 100 to 125 mg/dL signals prediabetes, and a reading that reaches 126 mg/dL on two separate occasions means you have diabetes.