Evaluation of HPA Axis Function in Pediatric Patients with Chronic Corticosteroid Therapy Using the Low-Dose (1 μg) Synacthen Test

Introduction: The low-dose Synacthen stimulation test (LD SST) is the gold standard for diagnosing adrenal insufficiency (AI) in children, but it requires multiple blood samples and is costly. This study aimed to identify cortisol thresholds predicting HPA axis response to LD SST. Patients & Methods: We studied 71 children on long-term corticosteroids. Patients were classified as having glucocorticoid-induced AI (G1, cortisol T30 and T60 < 18 µg/dL, n = 39) or normal adrenal function (G2, cortisol ≥ 18 µg/dL, n = 32). Results: There were 41 boys and 30 girls; 27 received oral corticosteroids and 44 inhaled corticosteroids. Mean morning cortisol was lower in G1 than G2 (6.69 ± 1.99 vs 9.21 ± 2.49 µg/dL, p < 0.001), and peak cortisol after stimulation was also lower (15.37 ± 1.38 vs 19.83 ± 3.01 µg/dL, p < 0.001). ROC analysis identified a morning cortisol < 6 µg/dL as predictive of AI (sensitivity 96.9%, specificity 46.2%), while > 13 µg/dL predicted normal HPA function (specificity 100%). Using these thresholds, 29.6% of LD SSTs could be avoided. Basal cortisol 6–13 µg/dL requires LD SST confirmation. An increment < 9 µg/dL at T30 suggests AI; ≥9 µg/dL likely excludes it. Conclusions: A morning cortisol threshold of 6 µg/dL can guide LD SST use, reducing costs and workload while maintaining diagnostic accuracy.