How are growth hormone (GH) levels tested?

Growth Hormone Level Testing Methods

Growth hormone (GH) levels are primarily tested through serum IGF-1 measurement and dynamic stimulation tests, with the oral glucose tolerance test being the most widely used provocative test for diagnosing GH excess. 1

Primary Testing Methods

Baseline Measurements

• Serum IGF-1 levels:
  • First-line screening test for GH disorders
  • Must be interpreted using age-adjusted, sex-adjusted, and Tanner stage-matched normal ranges 1
  • Elevated IGF-1 is a reliable marker for GH excess, though interpretation requires caution during adolescence 1
  • Very low IGF-1 levels in patients with high suspicion for GH deficiency may be diagnostic without further testing 2

Dynamic Testing for GH Excess

• Oral Glucose Tolerance Test (OGTT):
  • Gold standard for diagnosing GH excess
  • Measures GH suppression after glucose administration
  • In healthy adults, GH should suppress to below 1 μg/l (or 0.4 μg/l with sensitive assays) 1
  • In children/adolescents, suppression varies by sex and pubertal stage 1

Dynamic Testing for GH Deficiency

• Insulin Tolerance Test (ITT):

  • Test of choice for GH deficiency
  • Severe GH deficiency defined as GH peak <3 μg/ml 2
  • Contraindicated in patients with ischemic heart disease, seizure disorders, and elderly 2

• Alternative Stimulation Tests:

  • Glucagon Stimulation Test (GST):

    • GH typically peaks at 90 or 120 minutes
    • Test can be terminated at 150 minutes 3
    • Atypical timing of GH peak may indicate abnormal GH secretion 3

  • GHRH combined with arginine or GH secretagogues:

    • Well-defined cut-off limits
    • Reproducible and independent of age and gender 2

Important Considerations

Factors Affecting Test Results

• False normal/low IGF-1 in GH excess:

  • Severe hypothyroidism
  • Malnutrition
  • Severe infection 1

• False elevated IGF-1 without GH excess:

  • Poorly controlled diabetes mellitus
  • Hepatic/renal failure 1
  • Oral estrogens (can reduce IGF-1 generation) 1

• Body composition impact:

  • Overweight/obesity negatively correlates with GH response to all stimuli
  • Requires adjusted cut-offs to avoid false-positive diagnoses 2

Monitoring Considerations

• For patients receiving GH treatment, monitoring should include:
  • Height velocity
  • Pubertal development
  • Skeletal maturation (wrist radiography)
  • Thyroid hormone levels (TSH and free T3)
  • Metabolic parameters (glucose, calcium, phosphate, bicarbonate, PTH) 1

Clinical Context

• GH testing should be interpreted within the clinical context, including:

  • Height velocity
  • Pubertal stage
  • Bone age 1
  • Presence of associated syndromes (e.g., Carney complex, McCune-Albright syndrome, MEN1) 1

• In children with GH excess, dynamic assessment of other pituitary hormones is recommended due to potential co-secretion or hypofunction 1

GH testing requires specialized laboratory methods and careful interpretation by endocrinologists familiar with the nuances of these tests, as both false positive and false negative results can occur if testing protocols are not properly followed.