Hypoglycemia (low blood sugar)

Hypoglycemia is a blood glucose level low enough to cause symptoms, generally below about 3.9 mmol/L (70 mg/dL). Glucose is the brain's main fuel, so as it falls the body first releases adrenaline — causing shakiness, sweating, a fast heartbeat, hunger and anxiety — and, if the level drops further, the brain itself is affected, causing confusion, difficulty concentrating, slurred speech, drowsiness and, at the extreme, seizures or loss of consciousness. By far the commonest cause is treatment of diabetes: too much insulin or certain glucose-lowering tablets, a missed meal, extra exercise or alcohol. In people without diabetes it is much less common and prompts a search for a cause: certain medications, heavy alcohol use, severe illness of the liver, kidneys or heart, hormone deficiencies (such as low cortisol in adrenal insufficiency), and rarely a tumour that overproduces insulin (an insulinoma). A reactive form can occur a few hours after eating. The key clinical step, when a spontaneous low is confirmed, is to measure glucose, insulin and C-peptide together during an episode to work out why.

• Glucose — confirms the low level during symptoms (the crux of diagnosis).
• Insulin and C-peptide — measured together during a low: high insulin with high C-peptide suggests the body's own overproduction (e.g. insulinoma); high insulin with low C-peptide suggests injected insulin.
• Cortisol — checks for adrenal insufficiency as a cause.
• HbA1c — context in people with diabetes.
• Liver and kidney function — severe organ failure can cause lows.

• Shakiness and trembling
• Sweating and cold, clammy skin
• Hunger
• Palpitations or a fast heartbeat
• Anxiety or irritability
• Difficulty concentrating, confusion
• Dizziness, weakness
• Slurred speech, drowsiness
• If severe: seizures or loss of consciousness