Adrenal Insufficiency-related Outcomes in Patients with ACTH-stimulated Cortisol Levels of 15–17.9 μg/dL Measured by Monoclonal Antibody Assay

Introduction: The ACTH stimulation test helps diagnose adrenal insufficiency (AI), with a peak cortisol level above 18 μg/dL indicating normal adrenal function. Monoclonal antibody assays for cortisol measurement suggest a lower threshold of 15 μg/dL, but it remains unclear whether patients with cortisol levels between 15 and 17.9 μg/dL require glucocorticoid replacement therapy (GRT). Aim: To evaluate clinical outcomes of patients with peak cortisol levels of 15-17.9 μg/dL post-ACTH stimulation test who received or did not receive GRT. Methods: A retrospective study was conducted in adult patients who underwent ACTH stimulation tests with peak cortisol levels of 15-17.9 μg/dL and had at least 6-month follow-up. Serum cortisol levels were measured by monoclonal antibody assay. GRT decisions and AI diagnoses were determined by endocrine consultants. Results: Among 148 patients, 114 (77%) did not receive GRT. The mean basal cortisol level was 8.54 μg/dL. Both groups (GRT and non-GRT) had similar baseline characteristics, except for older age in the GRT group. During follow-up, 29 patients (26%) developed hyponatremia, mostly of mild degree. Both groups exhibited similar AI symptoms without severe AI-related hospital admission. Conclusion: Clinical outcomes in patients with cortisol levels of 15-17.9 μg/dL after ACTH stimulation were similar regardless of GRT use, although some differences in complete blood count existed. GRT may not be necessary for patients within this cortisol range following ACTH stimulation.