Type 2 diabetes almost always has a slow onset (often years), but, in type 1, particularly in children, onset may be quite fast (weeks or months). Early symptoms of type 1 diabetes are often polyuria (frequent urination) and polydipsia (increased thirst, and consequent increased fluid intake). There may also be weight loss (despite normal or increased eating), increased appetite, and irreducible fatigue. These symptoms may also manifest in type 2 diabetes in patients whose diabetes is poorly controlled.

Thirst develops because of osmotic effects—sufficiently high glucose (above the “renal threshold”) in the blood is excreted by the kidneys, but this requires water to carry it and causes increased fluid loss, which must be replaced. The lost blood volume will be replaced from water held inside body cells, causing dehydration.

Another common-presenting symptom is altered vision. Prolonged high blood glucose causes changes in the shape of the lens in the eye, leading to blurred vision and, perhaps, a visit to an optometrist. All unexplained quick changes in eyesight should force a fasting blood glucose test. These are now quick (10 seconds), using inexpensive (materials less than USD $1), and can be safely performed by almost anyone with minimal training.

Especially-dangerous symptoms in diabetics include the smell of acetone on the patient’s breath (a sign of ketoacidosis), Kussmaul breathing (a rapid, deep breathing), and any altered state of consciousness or arousal (hostility and mania are both possible, as is confusion and lethargy). The most dangerous form of altered consciousness is the so-called “diabetic coma,” which produces unconsciousness. Early symptoms of impending diabetic coma include polyuria, nausea, vomiting and abdominal pain, with lethargy and somnolence a later development, progressing to unconsciousness and death if untreated.