Background: Plasma insulin-like growth factor (IGF-I)
concentration can be used as a rough indicator of the
growth-hormone status. However, for the diagnosis of
growth hormone deficiency, dynamic tests are required.
The growth hormone (GH) response in the insulin
tolerance test (ITT) is considered to be the gold standard
in this respect. An alternative for the ITT is the GHRH/
GHRP-6 test, which has fewer side effects. In this study
we established reference values for IGF-I levels and for the GH response in both dynamic tests.
Methods: We studied 296 subjects recruited from the
general population, equally distributed according to sex
and aged between 20 and 70 years. Serum IGF-I level was measured in all subjects and an insulin tolerance test (0.15 U/kg Actrapid iv) and GHRH/GHRP-6 test (1 mg GHRH/kg and 1 mg GHRP-6/kg) were performed in 49 subjects.
Results: In multivariate analyses both IGF-I and the GH
response in the ITT were significantly influenced by age,
whereas the GH response in the GHRH/GHRP-6 test was
significantly affected by BMI. There was no sex difference in IGF-I and in the GHRH/GHRP-6 test, but in the ITT males had a higher GH peak. There was a significant correlation between the GH responses in both tests, and the GH response was significantly higher in the GHRH/GHRP-6 test than in the ITT. Age-adjusted reference values were established for each test.
Conclusion: We have established age-adjusted reference
values for serum IGF-I and for the GH response in the ITT and GHRH/GHRP-6 test.