Type 1 diabetes (T1D) becomes visible once enough insulin-producing beta cells have been lost that blood glucose can no longer be held down. The onset ranges from a gradual build-up of symptoms over days or weeks to the metabolic emergency of diabetic ketoacidosis. Recognising the picture matters, because the early symptoms are easy to dismiss until glucose is measured.
The symptoms of hyperglycemia
Glucose filtered by the kidney is normally reabsorbed completely, and once the filtered load exceeds what the proximal tubule can take back, the surplus is left in the urine and holds water with it. The result is an osmotic diuresis: urine volume rises, and with it the two symptoms that most often bring a patient to attention, polyuria (passing large volumes of urine, often waking at night) and polydipsia (excessive thirst). Because glucose cannot enter cells without insulin, the body breaks down fat and protein for fuel instead, so weight falls despite a normal or increased appetite (polyphagia), and fatigue is common. Osmotic shifts in the lens cause blurred vision that comes and goes with the glucose level. Polyuria, polydipsia and polyphagia are often named together as the three Ps of diabetes.
Diabetic ketoacidosis
When insulin deficiency becomes severe, fat is broken down and the liver converts the released fatty acids into ketone bodies (acetoacetate, beta-hydroxybutyrate and acetone). These are acids, so they accumulate as a metabolic acidosis. The breath smells of acetone, and breathing becomes deep and sighing (Kussmaul respiration) as the body blows off carbon dioxide to compensate. Nausea, vomiting and abdominal pain are frequent, dehydration and drowsiness follow, and untreated ketoacidosis progresses to coma and death. About two in five children with newly diagnosed type 1 diabetes, roughly 40%, are already in diabetic ketoacidosis at the time of diagnosis, so it should be suspected in any child who is vomiting, breathing deeply or generally unwell with a recent history of heavy urination and weight loss.
Age and mode of onset
Type 1 diabetes can begin at any age. In children, incidence peaks at about 10-14 years, but 25-50% of cases are diagnosed during adulthood, and up to 40% of adults older than 30 with type 1 diabetes may first be misdiagnosed with type 2 diabetes. The symptomatic onset is usually measured in days to weeks rather than months. Some people never present with symptoms at all, because autoantibody screening detects the disease while it is still pre-clinical and normoglycemic.
When to suspect type 1 diabetes
New polyuria, nocturia, weight loss, fatigue or blurred vision in a child, adolescent or young adult is the ordinary presentation, and a capillary blood glucose is the immediate test; blood or urine ketones should be checked in anyone who is unwell. A random plasma glucose of 200 mg/dL (11.1 mmol/L) or more together with classic symptoms is enough to confirm that diabetes is present, after which the autoimmune cause has to be established.