How do you interpret a growth hormone (GH) suppression test?

Interpreting Growth Hormone Suppression Test

The growth hormone (GH) suppression test should be interpreted by considering a GH nadir below 1 μg/l after oral glucose load as normal suppression in adults, while recognizing that children and adolescents have higher, age-specific, sex-specific and pubertal stage-dependent cutoff values. 1

Test Procedure and Normal Values

Adults

• Test involves administering a 75g oral glucose load and measuring GH levels at intervals
• Normal suppression in adults: GH nadir below 1 μg/l using modern sensitive assays 1
• Using older or less sensitive assays, cutoff may be higher (historical data showed ranges of 0.4-4.4 mIU/L) 2
• Failure to suppress below this threshold suggests GH excess/acromegaly

Children and Adolescents

• Higher cutoff values apply due to physiological differences
• Pubertal stage-specific thresholds:
  • Girls at Tanner stage 2-3 (mid-puberty): up to 1.57 ng/ml
  • Other pubertal stages in girls: up to 0.64 ng/ml
  • Boys (all pubertal stages): up to 0.50 ng/ml 3
• Almost all normal children suppress to less than 1.0 ng/ml 3
• Highest GH nadirs observed in mid-puberty, especially in girls 1

Key Considerations for Interpretation

Technical Factors

• Assay specificity and sensitivity are critical - use assay-specific reference ranges 1
• Modern, sensitive GH assays may require lower cutoff values (as low as 0.4 μg/l) 1
• Timing of samples is important - GH typically reaches nadir at 90-120 minutes 4

Physiological Factors

• Pubertal status significantly affects GH suppression 1, 3
• Gender differences exist (girls have higher nadir values than boys) 3
• Normal adolescents may fail to suppress GH adequately during puberty 1
• Approximately 30% of children with tall stature may not suppress GH below 1 μg/l 5

Clinical Context

• Interpret results alongside clinical assessment including:
  • Height velocity
  • Pubertal stage
  • Bone age
  • IGF-1 levels (elevated IGF-1 supports GH excess diagnosis) 1

Special Populations and Considerations

Acromegaly Diagnosis

• Acromegalic patients typically show:
  • Elevated fasting GH levels (though may overlap with normal in mild cases)
  • Failure to suppress or only partial suppression after glucose load 2
• Both GH and IGF-1 should be monitored at baseline and during follow-up 1

Transgender and Gender-Diverse Individuals

• For individuals on gender-affirming hormone therapy (GAHT) for >1 year, interpret laboratory values based on affirmed gender
• For those on GAHT <1 year, values may be in transition
• For those who have stopped GAHT, values typically reflect sex assigned at birth within 3 months 1

Pitfalls and Caveats

• False normal/low IGF-1 may occur with:

  • Severe hypothyroidism
  • Malnutrition
  • Severe infection 1

• False elevated IGF-1 may occur with:

  • Poorly controlled diabetes mellitus
  • Hepatic/renal failure 1

• Oral estrogens can reduce IGF-1 generation by the liver 1

• GH suppression test may not be reliable for:

  • Screening tall children (high false positive rate) 5
  • Follow-up of medically treated acromegaly patients 1

• Integrate test results with clinical picture - mild elevation during adolescent growth spurt needs cautious interpretation 1

Remember that the ultimate goal of identifying abnormal GH suppression is to diagnose conditions that affect morbidity, mortality, and quality of life, particularly acromegaly, which requires prompt treatment to prevent long-term complications.