A 40-year-old woman presents to the clinic for evaluation of symptoms of light-headedness associated with sweating, palpitations, and hunger. The symptoms are most pronounced whenever she misses a meal.On physical examination, her vital signs are normal, as is the heart, lungs, and abdominal examination. Her past medical history is negative and she is not taking any medications. During one such episode, while in hospital, her blood glucose level was 30 mg/dL and the symptoms resolved with drinking some juice. Which of the following is the most likely diagnosis?

Tumor of the pancreatic beta cells, such as an insulinoma, is the cause of her symptoms. Classification of hypoglycemia includes spontaneous causes such as reactive or fasting hypoglycemia and pharmacologic or toxic causes. The diagnosis of insulinoma is most certain when Whipple triad is fulfilled: symptoms consistent with hypoglycemia, low plasma glucose, and relief of symptoms with elevation of plasma glucose to normal. Cushing disease and excess growth hormone will result in elevated blood glucose levels and diabetes (not decreased glucose values). Thyrotoxicosis and gastrin deficiency do not cause hypoglycemia.