insulin tolerance test (ITT)

Indications

• evaluation of growth hormone deficiency
• evaluation of ACTH deficiency

Contraindications

• caution:
  • fatal hypoglycemia may occur
  • a physician must be present during testing Normal values:
    • serum glucose
      • baseline & 4 time points
      • falls to 50% of fasting level with 20-30 minutes
      • returns to baseline within 90-120 minutes
    • serum growth hormone (for growth hormone deficiency)
      • baseline & 4 time points
    • serum cortisol (for hypopituitarism, ACTH deficiency)
      • baseline & 4 time points

Adverse effects

• fatal hypoglycemia may occur
• a physician must be present during testing
• 50% glucose for IV injection should be available at the bedside

Clinical significance

• the insulin tolerance test is the gold standard for assessment of the hypothalamic-pituitary-adrenal axis, but is rarely needed
• it offers no advantage over the cosyntropin-stimulation test except in the first few days after acute ACTH deprivation
• increased tolerance
  • hypothyroidism
  • acromegaly
  • Cushing's syndrome
  • diabetes mellitus
• decreased tolerance
  • pancreatic islet cell tumor
  • adrenal insufficiency secondary to hypopituitarism
  • von Gierke disease
  • starvation
  • growth hormone deficiency

Procedure

• regular insulin is injected IV 0.1 units/kg
• serum glucose & cortisol levels are drawn at 0, 30, 60 & 90 minutes

Interpretation

• symptomatic hypoglycemia (< 40 mg/dL) is suggestive of adrenal insufficiency
• a peak cortisol level of > 20 ug/dL is normal

More general terms

• provocative clinical test
• special chemistry test

Additional terms

• Addison's disease (primary adrenal failure)
• adrenocorticotropic hormone; corticotropin; adrenocorticotropin (ACTH)
• aldosterone (Electrocortin, Aldocortin)
• cortisol; hydrocortisone (Cortef, Solu-Cortef, Alphaderm, Cetacort, Cortenema, Nutracort)
• cosyntropin (ACTH, Cortrosyn) stimulation test (delta cortisol test)
• glucose (dextrose, D-glucose)
• secondary adrenal insufficiency (SAI)