What is the most likely diagnosis for a patient with a glucose level of 7.2 mmol/L, insulin level of 3423 pmol/L, and C‑peptide level of 6201 pmol/L measured 30 minutes after a glucose tolerance test?

Insulinoma is the Most Likely Diagnosis

These laboratory values—markedly elevated insulin (3423 pmol/L) and C-peptide (6201 pmol/L) in the presence of a relatively normal glucose level (7.2 mmol/L) at 30 minutes post-glucose load—are pathognomonic for insulinoma, demonstrating inappropriate and excessive endogenous insulin secretion.

Diagnostic Reasoning

Why This is Insulinoma

The key diagnostic feature here is inappropriate insulin hypersecretion relative to the glucose level 1, 2:

• Insulin level of 3423 pmol/L (approximately 490 mcIU/mL) is profoundly elevated for a glucose of only 7.2 mmol/L (130 mg/dL) 1
• C-peptide of 6201 pmol/L confirms this is endogenous insulin production, not exogenous insulin administration 1, 2
• Both insulin AND C-peptide are simultaneously and dramatically elevated, which is the hallmark of insulinoma 1, 2

Diagnostic Criteria Met

According to NCCN guidelines, insulinoma is indicated when 1, 2:

• Insulin levels are inappropriately elevated (>3 mcIU/mL, usually >6 mcIU/mL) relative to glucose levels
• Elevated C-peptide levels confirm endogenous insulin production 1, 2
• The insulin-to-glucose ratio demonstrates inappropriate secretion 1

In this case, the insulin-to-glucose ratio is approximately 68 (3423 pmol/L ÷ 7.2 mmol/L × 0.144 conversion = ~68), which far exceeds the diagnostic threshold of 0.3 for insulinoma 1.

Why This is NOT Normal Glucose Tolerance

• Normal individuals would have much lower insulin and C-peptide levels at 30 minutes post-glucose load, typically insulin <600 pmol/L and C-peptide <2000 pmol/L 3, 4
• The glucose level of 7.2 mmol/L at 30 minutes is within normal range for an OGTT 5, but the insulin/C-peptide response is pathologically excessive 3, 4

Why This is NOT Type 2 Diabetes

• Type 2 diabetes patients may have elevated insulin due to insulin resistance, but not to this extreme degree 5
• The glucose level (7.2 mmol/L) does not meet diabetes criteria (<11.1 mmol/L at 2 hours) 6, 7, 6
• C-peptide levels in type 2 diabetes are typically elevated but not to 6201 pmol/L 8, 9

Next Steps for Confirmation and Management

Confirmatory Testing

1. Complete the 72-hour supervised fast (gold standard) to document Whipple's triad 1, 2:

   • Symptoms of hypoglycemia
   • Blood glucose <2.2-2.5 mmol/L (<40-45 mg/dL)
   • Insulin >30-40 pmol/L with C-peptide >200 pmol/L during hypoglycemia 10

2. Screen for sulfonylurea use via urinary testing to exclude factitious hypoglycemia 1, 2

Tumor Localization

Once biochemically confirmed 1, 2:

• Endoscopic ultrasound (EUS) is first-line, localizing ~82% of pancreatic neuroendocrine tumors 1, 2
• Multiphasic CT or MRI to assess for metastatic disease 1, 2
• Selective arterial calcium stimulation (Imamura-Doppman procedure) reserved for persistent/recurrent cases or when other localization is negative 1, 2

Important Caveats

• Avoid octreotide in insulinoma unless tumor is octreotide-avid on somatostatin scintigraphy, as it can paradoxically worsen hypoglycemia 1, 2
• Stabilize glucose with diet and/or diazoxide preoperatively 1, 2
• 90% of insulinomas are benign and surgically curable 1

Prognosis

Surgical resection offers excellent outcomes with potential cure in 90% of cases, as most insulinomas are solitary benign adenomas 1, 2.