Diabetes mellitus, often simply diabetes, is a syndrome characterized by disordered metabolism and inappropiately high blood sugar (hyperglycemia) resulting from either low levels of the hormone insulin or from abnormal resistance to insulin's effects coupled with inadequate levels of insulin secretion to compensate.[1] The characteristic symptoms are excessive urine production (polyuria), excessive thirst and increased fluid intake (polydipsia), and blurred vision; these symptoms may be absent if the blood sugar is mildly elevated.

The World Health Organization recognizes three main forms of diabetes mellitus: type 1, type 2, and gestational diabetes (occurring during pregnancy),[2] which have similar signs, symptoms, and consequences, but different causes and population distributions. Ultimately, all forms are due to the beta cells of the pancreas being unable to produce sufficient insulin to prevent hyperglycemia.[3] Type 1 diabetes is usually due to autoimmune destruction of the pancreatic beta cells, which produce insulin. Type 2 diabetes is characterized by insulin resistance in target tissues, but some impairment of beta cell function is necessary for its development. Gestational diabetes is similar to type 2 diabetes, in that it involves insulin resistance; the hormones of pregnancy can cause insulin resistance in women genetically predisposed to developing this condition.

Gestational diabetes typically resolves with delivery of the child, however types 1 and 2 diabetes are chronic conditions.[1] All types have been treatable since insulin became medically available in 1921. Type 1 diabetes, in which insulin is not secreted by the pancreas, is directly treatable only with injected or inhaled insulin, although dietary and other lifestyle adjustments are part of management. Type 2 may be managed with a combination of dietary treatment, tablets and injections and, frequently, insulin supplementation. While insulin was originally produced from natural sources such as porcine pancreas, most insulin used today is produced through genetic engineering, either as a direct copy of human insulin, or human insulin with modified molecules that provide different onset and duration of action. Insulin can also be delivered continuously by a pump surgically embedded under the skin.