Hyperinsulinism describes the clinical consequences of too much insulin action — classically the diabetic’s reaction to an excessive insulin dose, or to too little food for the dose taken. Blood sugar plunges into hypoglycaemia: shakiness, sweating, confusion, and hunger progressing to unconsciousness and seizures if untreated — the “insulin reaction” every insulin-treated diabetic learns to fear and to treat with fast-acting glucose. Beyond diabetes, hyperinsulinism occurs with insulin-secreting tumours (insulinomas) and in newborns of diabetic mothers. The term emphasises the syndrome — the body’s crisis under insulin excess — rather than the blood measurement (hyperinsulinaemia). Management is immediate glucose, then finding and fixing the cause of the overshoot.