An insulinoma is a rare tumor of the pancreas that produces too much insulin, causing your blood sugar to drop dangerously low. These tumors grow in the insulin-producing cells of the pancreas and affect roughly 1 to 4 people per million each year. The good news: about 87% of insulinomas are benign, and surgery cures most cases.
How an Insulinoma Disrupts Blood Sugar
Normally, your pancreas releases insulin only when blood sugar rises, like after a meal. Insulin moves sugar from the blood into cells for energy, and once blood sugar returns to a normal range, insulin secretion stops. An insulinoma ignores that feedback loop. The tumor keeps pumping out insulin regardless of how low your blood sugar gets, creating a condition called hyperinsulinemic hypoglycemia.
This is different from what happens in diabetes, where the problem is too little insulin or insulin that doesn’t work well. With an insulinoma, the problem is the opposite: uncontrolled insulin production that starves your brain and body of glucose.
Symptoms of Low Blood Sugar
The symptoms of an insulinoma are really the symptoms of repeated low blood sugar episodes. They tend to appear when you haven’t eaten for several hours, often in the early morning or between meals. The pattern falls into two categories: what your body does to compensate for low sugar, and what happens when your brain doesn’t get enough fuel.
The compensatory response triggers sweating, shakiness, a racing heart, anxiety, and intense hunger. These are your body’s alarm signals, driven by adrenaline. If blood sugar keeps falling, typically below 50 to 55 mg/dL, brain-related symptoms take over: confusion, difficulty concentrating, blurred vision, slurred speech, unusual behavior, and in severe cases, seizures or loss of consciousness.
A hallmark pattern doctors look for is called Whipple’s triad, first described in 1938: symptoms consistent with low blood sugar, a confirmed low glucose reading (below 70 mg/dL), and immediate relief of symptoms once blood sugar is corrected. Many people with insulinomas learn instinctively that eating something sugary stops the episodes, which can mask the problem for months or even years. Weight gain is common because frequent snacking becomes a survival strategy.
Who Gets Insulinomas
Insulinomas can appear at any age, from teenagers to people in their 80s, but diagnosis peaks during the 40s and 50s. The average age at diagnosis is around 56. Men and women are affected at roughly equal rates, though one Swedish study found women tended to be diagnosed about a decade later than men, at an average age of 63 compared to 50.
Most insulinomas occur on their own with no known genetic cause. However, about 4% to 6% of people with an insulinoma turn out to have a hereditary condition called Multiple Endocrine Neoplasia type 1 (MEN1), which causes tumors in multiple hormone-producing glands. If you’re diagnosed with an insulinoma, your doctor may check for MEN1, especially if you have a family history of endocrine tumors or if you develop more than one insulinoma.
How Insulinomas Are Diagnosed
Diagnosing an insulinoma starts with proving that your body is making too much insulin at the wrong time. The gold standard test is a supervised fast lasting up to 72 hours, performed in a hospital. During the fast, your blood is drawn at regular intervals to measure glucose, insulin, and related markers. The test is considered positive when blood sugar drops below 55 mg/dL while insulin levels remain inappropriately elevated rather than shutting off as they should.
Most people with insulinomas develop symptoms within 24 to 48 hours of fasting, though some need the full 72 hours. The test also helps rule out other causes of low blood sugar, such as certain medications or other metabolic conditions.
Finding the Tumor
Once blood work confirms the diagnosis, the next step is locating the tumor. Insulinomas are often small, sometimes less than a centimeter across, which makes them tricky to find on imaging. Standard ultrasound picks them up only 25% to 60% of the time. CT scans do somewhat better, with sensitivity ranging from 30% to 80% depending on tumor size and characteristics.
MRI performs considerably better, detecting insulinomas with about 93% sensitivity. Endoscopic ultrasound, where a small ultrasound probe is passed through the mouth into the stomach to get a close-up view of the pancreas, matches that sensitivity at 85% to 94% and is considered the most reliable tool for finding small tumors. Many patients undergo more than one type of imaging before the tumor is pinpointed.
Treatment: Surgery Is the Primary Cure
Surgery is the definitive treatment for insulinomas and cures the vast majority of patients with benign tumors. The type of operation depends on where the tumor sits within the pancreas and how close it is to the main pancreatic duct.
For small, superficial tumors that sit away from the pancreatic duct, surgeons can often perform an enucleation, essentially shelling out just the tumor while leaving the rest of the pancreas intact. This is a less invasive option with a quicker recovery. When the tumor is embedded deeper in the pancreas or sits close to the duct, a more formal resection is needed, removing a section of the pancreas along with the tumor. Both approaches can often be done laparoscopically, meaning smaller incisions and shorter hospital stays.
For patients with benign tumors who undergo surgery, survival rates are excellent, reaching 100% in some studies. The picture is very different for the roughly 13% of insulinomas that turn out to be malignant. Malignant insulinomas carry a much worse outlook, with one retrospective study reporting a five-year survival rate of only 24% and insulinoma-related death in 77% of those patients over follow-up periods ranging from one month to 25 years.
Managing Symptoms Before or Without Surgery
For patients waiting for surgery or those who can’t undergo an operation, medication can help control blood sugar. The most commonly used drug works by opening specific channels in insulin-producing cells, which reduces insulin release. This effect is reversible and can keep blood sugar from dropping to dangerous levels.
Frequent small meals, especially ones rich in complex carbohydrates, also help smooth out blood sugar swings. Some patients carry glucose tablets or sugary drinks to treat sudden episodes. These strategies manage symptoms but don’t address the tumor itself, making surgery the preferred approach whenever possible.
Benign vs. Malignant Insulinomas
The single most important factor in long-term outlook is whether the tumor is benign or malignant. Benign insulinomas, which account for about 87% of cases, are typically small, solitary, and confined to the pancreas. Once removed, they rarely come back. Malignant insulinomas may have already spread to the liver or nearby lymph nodes by the time they’re found. Signs that raise suspicion for malignancy include larger tumor size and evidence of spread on imaging.
There is no reliable way to distinguish benign from malignant insulinomas before surgery based on symptoms alone, since both types cause the same low blood sugar episodes. The distinction is usually made by a pathologist examining the removed tissue, or by the presence of metastases on preoperative imaging.