The classic symptoms of untreated diabetes are polyuria, thirst, and weight loss.^[20] Several other non-specific signs and symptoms may also occur, including fatigue, blurred vision, and genital itchiness due to Candida infection.^[20] About half of affected individuals may also be asymptomatic.^[20] Type 1 presents abruptly following a pre-clinical phase, while type 2 has a more insidious onset; patients may remain asymptomatic for many years.^[21]

Diabetic ketoacidosis is a medical emergency that occurs most commonly in type 1, but may also occur in type 2 if it has been longstanding or if the individual has significant β-cell dysfunction.^[22] Excessive production of ketone bodies leads to signs and symptoms including nausea, vomiting, abdominal pain, the smell of acetone in the breath, deep breathing known as Kussmaul breathing, and in severe cases decreased level of consciousness.^[22] Hyperosmolar hyperglycemic state is another emergency characterized by dehydration secondary to severe hyperglycemia, with resultant hypernatremia leading to an altered mental state and possibly coma.^[23]

Hypoglycemia is a recognized complication of insulin treatment used in diabetes.^[24] An acute presentation can include mild symptoms such as sweating, trembling, and palpitations, to more serious effects including impaired cognition, confusion, seizures, coma, and rarely death.^[24] Recurrent hypoglycemic episodes may lower the glycemic threshold at which symptoms occur, meaning mild symptoms may not appear before cognitive deterioration begins to occur.^[24]