SUN-473 Clinical Utility of the 8-Mg-Dexamethasone Suppression Test in Assessing Hypercortisolism

Abstract Disclosure: M. Salehi: None. K. Yu: None. C.A. Villavicencio Torres: None. V. Fell: None. I. Bancos: None. Context: The use of the 1 mg overnight dexamethasone suppression test (1 mg-DST) is considered the standard-of-care test for the assessment of hypercortisolism. Scarce data are available on the utility of 8 mg-DST in distinguishing adrenal from pituitary hypercortisolism. Objective: To determine the value of 8 mg-DST in diagnosing hypercortisolism subtype. Methods: We conducted a single-center retrospective cohort study (2005-2024) of consecutive adults who underwent both the 1 mg-DST and the 8 mg-DST within a 6-month period. Only patients with optimal reference standard confirming the type of hypercortisolism were included. Patients were excluded if they had an abnormal DST and dexamethasone concentration <100 ng/dL or were receiving oral estrogen therapy. Delta cortisol [(post 1 mg-DST cortisol- post 8 mg-DST cortisol) x100/post 1 mg-DST cortisol)] was calculated for each patient. Results: A total of 61 patients (50, 82% women, median age 60 years, interquartile range, IQR 47-67) underwent both 1- and 8 mg-DST at least once, with 8 (13%) patients undergoing the 1 mg-DST and 8 mg-DST twice, and 1 (1.6%) patient undergoing both tests thrice. Adrenal hypercortisolism was diagnosed in 44 (72%) patients and pituitary hypercortisolism in 17 (28%) patients. Median post 1 mg-DST cortisol was 4.6 mcg/dL (IQR 2.8-7.2) in adrenal and 13.7 mcg/dL (IQR 7.3-21.7) in pituitary hypercortisolism. Median post 8 mg-DST cortisol was 3.2 mcg/dL (IQR 2.5-6.3) in adrenal and 3.6 mcg/dL (IQR 2.2-7.5) in pituitary hypercortisolism. Median delta cortisol was higher in pituitary hypercortisolism when compared to adrenal hypercortisolism (69%, IQR 25-83 vs 5 %, IQR -20-16, P<0.001). Delta cortisol demonstrated a good discrimination in distinguishing adrenal from pituitary hypercortisolism with a median area under receiver operator of 82% (95%CI 69-92). A cut-off of 25% provided a specificity of > 85%, while a cut-off of -1% yielded a sensitivity of > 85% in diagnosing adrenal hypercortisolism. Conclusion: In this study, we show that performing 8 mg-DST in addition to 1 mg-DST in selected patients with hypercortisolism and calculating delta cortisol is a valuable in distinguishing hypercortisolism subtype. Presentation: Sunday, July 13, 2025