What are the differential diagnoses of spontaneous (non‑iatrogenic) hypoglycaemia?

Differential Diagnosis of Spontaneous Hypoglycemia

The differential diagnosis of spontaneous hypoglycemia in non-diabetic adults systematically divides into three major categories: drugs (most common), critical illnesses with hormone deficiencies, and endogenous hyperinsulinism, with the evaluation pathway determined by whether the patient appears ill or well 1.

Primary Categories

1. Drug-Induced Hypoglycemia (Most Common)

• Exogenous insulin or insulin secretagogues (sulfonylureas, repaglinide)
  • Includes accidental, surreptitious, or malicious administration 1
  • Pharmacy errors (medication substitution) 1
  • More common in individuals with access to glucose-lowering medications 1
• Alcohol - inhibits gluconeogenesis, occurs in fasting states 2
• Other medications - various drugs beyond antidiabetic agents 1

2. Critical Illnesses and Hormone Deficiencies

• Sepsis and critical illness 1
• Organ failure: renal or hepatic failure 1
• Adrenocortical insufficiency - though rarely found without other clinical clues 1
• Hypopituitarism (growth hormone or cortisol deficiency) 2

3. Non-Islet Cell Tumor Hypoglycemia

• Large mesenchymal tumors producing incompletely processed IGF-II (pro-IGF-II) 1
• Characterized by:
  • Elevated pro-IGF-II to IGF-II ratio
  • Elevated IGF-II to IGF-I ratio (due to suppressed GH)
  • Appropriately suppressed endogenous insulin secretion 1

4. Endogenous Hyperinsulinism (In Seemingly Well Individuals)

This category requires the most systematic evaluation:

A. Insulinoma

• Incidence: 1 in 250,000 patient-years 1
• Presentation: Primarily fasting hypoglycemia, occasionally postprandial 1
• Demographics: Slight female predominance, any age or ethnicity 1
• Characteristics: <10% malignant, <10% multiple tumors or MEN-1 syndrome 1
• Recurrence: 7% without MEN-1,21% with MEN-1 1

B. Noninsulinoma Pancreatogenous Hypoglycemia Syndrome (NIPHS)

• Presentation: Typically postprandial (but not invariably) 1
• Demographics: Male predominance 1
• Pathology: Diffuse nesidioblastosis (islet hypertrophy/hyperplasia) 1
• Imaging: Invariably negative 1
• Frequency: Much less common than insulinoma 1

C. Post-Gastric Bypass Hypoglycemia

• Timing: Months after Roux-en-Y gastric bypass surgery 1
• Presentation: Usually postprandial with nesidioblastosis; fasting if insulinoma 1
• Demographics: Female predominance (reflects bariatric surgery demographics) 1
• Incidence: At Mayo Clinic, exceeds insulinoma cases 1

D. Insulin Autoimmune Hypoglycemia (Hirata's Syndrome)

• Demographics: Primarily Japanese or Korean ethnicity, rare in Caucasians 1
• Presentation: Late postprandial hypoglycemia 1
• Mechanism: Insulin bound to antibodies dissociates in unregulated fashion 1
• Associated factors: Autoimmune disease history, sulfhydryl-containing drug exposure 1
• Diagnostic clue: Very high measured insulin levels (may be assay artifact) 1
• Confirmation: High titer serum insulin antibodies 1

Critical Diagnostic Approach

When evaluating a seemingly well individual, first exclude accidental, surreptitious, or malicious hypoglycemia before pursuing endogenous hyperinsulinism workup 1.

Key Laboratory Criteria During Hypoglycemia

Endogenous hyperinsulinism is diagnosed when plasma glucose <55 mg/dL (3.0 mmol/L) occurs with:

• Insulin ≥3.0 μU/mL (18 pmol/L)
• C-peptide ≥0.6 ng/mL (0.2 nmol/L)
• Proinsulin ≥5.0 pmol/L
• β-hydroxybutyrate ≤2.7 mmol/L
• Plasma glucose increase ≥25 mg/dL (1.4 mmol/L) after 1.0 mg IV glucagon 1

Important Caveats

Normal insulin secretion is virtually completely suppressed during hypoglycemia - insulin <3 μU/mL, C-peptide <0.6 ng/mL, proinsulin <5.0 pmol/L 1. Values that appear "normal" in the euglycemic range are inappropriately elevated when glucose is <55 mg/dL and indicate hyperinsulinism 1.

A low cortisol during spontaneous hypoglycemia does not confirm adrenocortical insufficiency - recurrent hypoglycemia shifts glycemic thresholds for cortisol secretion lower 1.

The traditional fasting versus postprandial classification has limitations: insulinoma patients may have postprandial symptoms, and post-gastric-bypass patients may have fasting symptoms 1.

Rare Causes

• Agonist antibodies to insulin receptor - typically African-American females with autoimmune disease; inappropriately elevated insulin with suppressed C-peptide 1
• Nesidioblastosis from prolonged sulfonylurea use 1
• Ectopic insulin secretion - extremely rare 1
• Insulin receptor mutation - rare 1
• Exercise-induced hyperinsulinemia - rare 1