A person's body regulates fluid by balancing liquid intake and removing extra fluid. Thirst usually controls a person’s rate of liquid intake, while urination removes most fluid, although people also lose fluid through sweating, breathing, or diarrhea. The hormone vasopressin, also called antidiuretic hormone, controls the fluid removal rate through urination. The hypothalamus, a small gland located at the base of the brain, produces vasopressin. The nearby pituitary gland stores the vasopressin and releases it into the bloodstream when the body has a low fluid level. Vasopressin signals the kidneys to absorb less fluid from the bloodstream, resulting in less urine. When the body has extra fluid, the pituitary gland releases smaller amounts of vasopressin, and sometimes none, so the kidneys remove more fluid from the bloodstream and produce more urine.
The classic symptoms of untreated diabetes are unintended weight loss, polyuria (increased urination), polydipsia (increased thirst), and polyphagia (increased hunger).[22] Symptoms may develop rapidly (weeks or months) in type 1 diabetes, while they usually develop much more slowly and may be subtle or absent in type 2 diabetes. Other symptoms of diabetes include weight loss and tiredness.[23]

The American Diabetes Association sponsored an international panel in 1995 to review the literature and recommend updates of the classification of diabetes mellitus. The definitions and descriptions that follow are drawn from the Report of the Expert Committee on the Diagnosis and Classification of Diabetes Mellitus. The report was first approved in 1997 and modified in 1999. Although other terms are found in older literature and remain in use, their use in current clinical practice is inappropriate. Epidemiologic and research studies are facilitated by use of a common language.

Since cardiovascular disease is a serious complication associated with diabetes, some have recommended blood pressure levels below 130/80 mmHg.[92] However, evidence supports less than or equal to somewhere between 140/90 mmHg to 160/100 mmHg; the only additional benefit found for blood pressure targets beneath this range was an isolated decrease in stroke risk, and this was accompanied by an increased risk of other serious adverse events.[93][94] A 2016 review found potential harm to treating lower than 140 mmHg.[95] Among medications that lower blood pressure, angiotensin converting enzyme inhibitors (ACEIs) improve outcomes in those with DM while the similar medications angiotensin receptor blockers (ARBs) do not.[96] Aspirin is also recommended for people with cardiovascular problems, however routine use of aspirin has not been found to improve outcomes in uncomplicated diabetes.[97]
At the same time that the body is trying to get rid of glucose from the blood, the cells are starving for glucose and sending signals to the body to eat more food, thus making patients extremely hungry. To provide energy for the starving cells, the body also tries to convert fats and proteins to glucose. The breakdown of fats and proteins for energy causes acid compounds called ketones to form in the blood. Ketones also will be excreted in the urine. As ketones build up in the blood, a condition called ketoacidosis can occur. This condition can be life threatening if left untreated, leading to coma and death.