What is the protocol for a growth hormone (GH) suppression test?

Growth Hormone Suppression Test Protocol

The standard protocol for growth hormone (GH) suppression testing involves administering an oral glucose load of 75g and measuring GH levels at baseline and at multiple time points afterward, with a GH nadir below 1 μg/l indicating normal suppression in adults. 1

Test Preparation and Administration

• Patients should be fasting prior to the test 2
• Administer 75g oral glucose load (standard dose for adults) 1, 2
• For children, the dose is weight-adjusted at 1.75 g/kg (maximum 75g) or 2.35 g/kg (maximum 100g) depending on institutional protocol 1
• Evening subcutaneous injections are recommended if this is part of GH therapy monitoring 3

Blood Sampling Schedule

• Obtain baseline GH and IGF-1 levels before glucose administration 1
• Collect blood samples at multiple time points after glucose administration:
  • The optimal combination of time points for maximum accuracy (100%) includes samples at 60,90,120, and 150 minutes 4
  • The 90-minute sample is most frequently associated with maximum GH suppression in healthy individuals 4
  • If limited sampling is necessary, the combination of 90 and 150 minutes achieves 95.6% accuracy 4

Interpretation of Results

• In healthy adults, GH should suppress to below 1 μg/l (using standard assays) or 0.4 μg/l (using sensitive GH assays) 1
• Sex-specific reference ranges should be considered, as women typically have higher nadir GH values than men 5:
  • For men: 0.14-0.25 μg/l (depending on assay)
  • For women: 0.25-0.70 μg/l (depending on assay) 5
• In children and adolescents, interpretation is more complex:
  • GH suppression is sex and pubertal stage specific 1
  • Highest levels occur in mid-puberty (Tanner stage 2-3), particularly in girls 1
  • Approximately 30% of children with tall stature may not show GH suppression below 1 μg/l 1

Special Considerations

• Failure to suppress GH after glucose load is diagnostic of GH excess, but must be interpreted alongside clinical features and IGF-1 levels 1
• IGF-1 values might be falsely normal or low in patients with GH-secreting adenoma who have concurrent severe hypothyroidism, malnutrition, or severe infection 1
• IGF-1 may be falsely elevated in patients without GH-secreting adenoma who have poorly controlled diabetes mellitus, hepatic and/or renal failure 1
• Oral estrogens can reduce IGF-1 generation by the liver, potentially confounding test results 1
• In patients on somatostatin analog treatment, GH suppression testing may be less reliable and show discordance with IGF-1 levels 6, 7

Additional Monitoring During GH Therapy

• Monitor for potential side effects if the patient is receiving GH therapy:
  • Intracranial hypertension (rare but serious) 3
  • Glucose metabolism alterations 3
  • Secondary hyperparathyroidism 3
  • Lipoatrophy at injection sites 3
• Regular clinic visits (every 3-6 months) are recommended to monitor growth parameters, pubertal development, skeletal maturation, and metabolic parameters 1, 3

Pitfalls and Caveats

• Assay variability can significantly impact results; laboratories should establish their own reference ranges 5
• Test repetition shows variation in nadir GH of approximately 10-13% in 95% of subjects 5
• In acromegalic patients, elevated fasting GH levels may overlap with normal ranges in some cases, making the suppression test essential for accurate diagnosis 2
• Discordance between GH nadir and IGF-1 levels occurs in approximately 33% of postoperative assessments without medication, highlighting the importance of measuring both parameters 7