Several forms of reactive hypoglycemia occur. In a diabetic patient, it may result from administration of too much insulin or, less commonly, too much oral antidiabetic medication. In a mildly diabetic patient (or one in the early stages of diabetes mellitus), reactive hypoglycemia may result from delayed and excessive insulin production after carbohydrate ingestion.

Similarly, a nondiabetic patient may suffer reactive hypoglycemia from a sharp increase in insulin output after a meal. Sometimes called postprandial hypoglycemia, this type of reactive hypoglycemia usually disappears when the patient eats something sweet.

In some patients, reactive hypoglycemia may have no known cause (idiopathic reactive) or may result from total parenteral nutrition due to gastric dumping syndrome or from impaired glucose tolerance.

Fasting hypoglycemia usually results from an excess of insulin or insulin-like substance or from a decrease in counterregulatory hormones. It can be exogenous, resulting from such external factors as alcohol or drug ingestion, or endogenous, resulting from organic problems.

Endogenous hypoglycemia may result from tumors or liver disease. Insulinomas, small islet cell tumors in the pancreas, secrete excessive amounts of insulin, which inhibits hepatic glucose production. They’re generally benign (in 90% of patients).