Challenges in the interpretation of cortisol response during insulin tolerance test: prevalence of secondary adrenal insufficiency and predictive factors

Background: The insulin tolerance test (ITT) is the gold standard for assessing the integrity of growth hormone (GH) and cortisol axes. While ITT is essential for diagnosing growth hormone deficiency (GHD) and secondary adrenal insufficiency (SAI), its interpretation can be challenging due to variability in thresholds and individual responses. Methods: This retrospective study analyzed ITTs performed at the Institute for Maternal and Child Health IRCCS “Burlo Garofolo,” Trieste, Italy, from January 1, 2019, to December 31, 2024. Tests were conducted to confirm GHD after a blunted response to arginine stimulation or for retesting at the end of GH treatment. Regular insulin (0.1 IU/kg intravenously) was administered, and adequate hypoglycemia was defined as a S50% decrease in basal glucose or a nadir glucose <40 mg/dL. Results: Of 212 ITTs performed, adequate hypoglycemia was achieved in 186 (88%), including 157 diagnostic tests and 29 retests. The prevalence of SAI varied widely depending on the threshold used: Between 14% and 27% of individuals meeting these thresholds were not diagnosed with GHD. Peak cortisol levels positively correlated with basal cortisol (r2=0.490, P<.001), nadir glycemia (r2=0.307, P<.001), basal ACTH (r20.255, P<.001), ACTH after hypoglycemia (r2=0.332, P<.001), and peak GH (r2=0.312, P< .001). No significant correlation was found with age, sex, or BMI SDS.Multivariate analysis identified lower basal cortisol (P <.001), lower ACTH after hypoglycemia (P<.001), and nadir glycemia (P<.012) as significant predictors of cortisol peak, with a moderate model fit (R2=0.354). Conclusions: The prevalence of SAI during ITT ranged from 13% to 80% depending on the diagnostic thresholds. Between 14% and 27% of individuals meeting SAI thresholds were not diagnosed with GHD, complicating interpretation. Significant predictors of cortisol peak include basal cortisol, nadir glycemia, and ACTH after hypoglycemia. These findings highlight the need to consider clinical context alongside hormonal thresholds to ensure accurate SAI diagnosis and avoid overdiagnosis.