From the Guidelines
The growth hormone (GH) suppression test should be performed by administering an oral glucose load, with a cut-off GH nadir of 1 μg/l, to diagnose GH excess in children and adolescents, as recommended by the consensus guideline for the diagnosis and management of pituitary adenomas in childhood and adolescence 1. The protocol for the GH suppression test involves administering an oral glucose load, typically 1.75 g/kg, maximum 75 g, to a fasting patient and measuring GH levels at baseline and at 30-minute intervals for 2 hours.
- Patients should fast for 8-12 hours before the test and remain seated or recumbent during testing.
- Blood samples are typically collected at 0,30,60,90, and 120 minutes after glucose ingestion.
- In normal individuals, glucose administration suppresses GH to less than 1 μg/l, and failure of GH to suppress below this level suggests autonomous GH secretion, as seen in acromegaly. The test works because glucose normally inhibits GH secretion through increased insulin levels, but GH-secreting pituitary adenomas are resistant to this inhibition.
- Patients should avoid exercise, stress, and medications that affect GH levels, such as dopamine agonists, somatostatin analogs, or GH receptor antagonists, before testing. This test is particularly useful for diagnosing acromegaly when GH levels are borderline elevated or when IGF-1 levels are equivocal, as elevated serum IGF1 concentration is a reliable marker for GH excess 1. However, it is essential to interpret biochemical results within a clinical assessment of phenotype, including height velocity, pubertal stage, and bone age, as GH nadir after glucose load in children and adolescents undergoing puberty is sex and pubertal stage specific 1.
From the Research
Growth Hormone Suppression Test Protocol
The growth hormone (GH) suppression test, also known as the oral glucose tolerance test (OGTT), is used to evaluate the secretion of GH in response to glucose ingestion. The protocol for this test involves:
- Administering a glucose load, typically 75g, to the patient
- Measuring GH levels at various time points after glucose ingestion, usually at 30,60,90,120, and 180 minutes
- Evaluating the nadir GH level, which is the lowest GH concentration reached during the test
Timing of Blood Samplings
The optimal timing of blood samplings to detect GH inhibition during OGTT is still a matter of debate. However, studies suggest that:
- Sampling at 90 minutes is the most frequent time point at which a GH level < 1 μg/L is achieved, in both healthy subjects and cured acromegalic patients 2
- The best combination of 2 time-points is +90' and +150', which achieves 95.6% accuracy 2
- The best combination of 3 time-points is +60', +90', and +150', which achieves 99.1% accuracy 2
- A subset of 4 time-points, namely +60', +90', +120', and +150', is identified as the most relevant to detect GH suppression at OGTT, with a perfect classification of 100% of subjects 2
Interpretation of Results
The results of the GH suppression test are interpreted as follows:
- A nadir GH level < 1 μg/L is considered normal suppression 3, 4
- A nadir GH level ≥ 1 μg/L is considered nonsuppression, which may indicate acromegaly or other disorders of GH secretion 3, 4
- The test is not recommended as a general screening for excess GH in tall children and adolescents, as absent suppression of GH by oral glucose is common in this population 5
Factors Affecting GH Suppression
Several factors can affect GH suppression during OGTT, including: