GHRP-2 test predicts secondary hypoadrenalism with 89.7% sensitivity and 88.9% specificity
Background
Diagnosing secondary adrenal failure, a condition of insufficient cortisol production due to disorders of the pituitary or hypothalamus, is a significant clinical challenge. The gold standard for assessing the hypothalamo-pituitary-adrenal (HPA) axis is the insulin tolerance test (ITT), but it carries risks and is contraindicated in some patients. Alternative methods like the CRH stimulation test exist, but there is a need for safer and more reliable diagnostic tools. Growth hormone-releasing peptide-2 (GHRP-2) is known to stimulate ACTH and cortisol secretion, suggesting it could be a valuable probe for HPA axis integrity. This study investigates whether the GHRP-2 test can be a clinically useful alternative for diagnosing HPA axis failure in patients with suspected hypothalamo-pituitary disorders.
Study Design
The study involved 47 patients with suspected hypothalamo-pituitary disorders and a control group. Each participant underwent three separate tests: an insulin tolerance test (ITT), a corticotropin-releasing hormone (CRH) stimulation test, and a GHRP-2 test. The GHRP-2 test involved administering the peptide and measuring subsequent serum levels of adrenocorticotropic hormone (ACTH) and cortisol. The primary goal was to determine if the GHRP-2 test could accurately predict secondary hypoadrenalism by comparing ACTH and cortisol responses between patient subgroups (hypothalamic vs. pituitary disorders) and the control group, and establishing a diagnostic cut-off value.
Results
Patients with pituitary disorders had significantly lower ACTH responses to the GHRP-2 test compared to patients with hypothalamic disorders and controls. However, peak cortisol levels were significantly lower in response to GHRP-2 in both hypothalamic and pituitary disorder groups compared to the control group. The study identified a specific cut-off value for peak serum cortisol that optimized diagnostic accuracy.
A cut-off value of 11.6 μg/dL for peak serum cortisol during the GHRP-2 test predicted secondary hypoadrenalism with 88.9% specificity and 89.7% sensitivity.
Interestingly, the ACTH and cortisol responses induced by GHRP-2 showed no correlation with the responses seen in the CRH test. This suggests that GHRP-2 stimulates ACTH secretion through a different, more direct mechanism on the pituitary gland, rather than acting through the CRH pathway. The diagnostic potential of the GHRP-2 test was found to be similar to that of the CRH stimulation test for evaluating HPA axis failure.
Key Findings
- A peak cortisol cut-off of 11.6 μg/dL in the GHRP-2 test diagnoses secondary hypoadrenalism with 89.7% sensitivity.
- The same 11.6 μg/dL cortisol cut-off demonstrated 88.9% specificity for diagnosing secondary hypoadrenalism.
- GHRP-2 induced ACTH and cortisol responses had no correlation with responses to the CRH test, suggesting a different mechanism.
- The diagnostic potential of the GHRP-2 test is similar to the CRH stimulation test for HPA axis failure.
- Patients with pituitary disorders had significantly lower ACTH responses to GHRP-2 than those with hypothalamic disorders or controls.
Why It Matters
The GHRP-2 test emerges as a viable diagnostic tool for secondary adrenal failure, offering diagnostic power comparable to the established CRH stimulation test. For clinicians, this provides another option for assessing the HPA axis, particularly when the gold-standard ITT is too risky. The key implication is that a simple GHRP-2 administration followed by a cortisol measurement could reliably screen for adrenal insufficiency. This establishes a specific protocol: a peak cortisol level below 11.6 μg/dL after GHRP-2 administration strongly suggests HPA axis failure. The finding that GHRP-2 acts directly on the pituitary to release ACTH, independent of CRH, also deepens our understanding of pituitary regulation and could inform the development of more targeted diagnostic tests for distinguishing between hypothalamic and pituitary sources of adrenal insufficiency.
