To compare clinical characteristics, biochemical parameters, and tumor markers between patients with Cushing disease (CD) and ectopic adrenocorticotropic hormone (ACTH) syndrome (EAS), identify potential biomarkers, and optimize their diagnostic differentiation.;A single-center retrospective study was conducted in 75 patients with ACTH-dependent Cushing syndrome (51 CD, 24 EAS) admitted between January 2006 and April 2025. Differences in biochemical indices, tumor markers, and hormone levels were analyzed. Receiver operating characteristic (ROC) analysis was used to evaluate diagnostic performance and determine optimal cut-off values.;EAS patients were significantly older at diagnosis than CD patients. Fasting glucose was higher, while serum potassium and albumin were lower in the EAS group. Non-small cell lung cancer-associated antigen, carcinoembryonic antigen (CEA), cancer antigen 15-3 (CA15-3), and cancer antigen 19-9 were markedly elevated in EAS, whereas pro-gastrin-releasing peptide (ProGRP) and neuron-specific enolase (NSE) showed no significant differences. Serum cortisol and plasma ACTH levels at all time points and after dexamethasone suppression were significantly higher in EAS. ROC analysis of the product of midnight serum cortisol (μg/dL) and plasma ACTH (pg/mL) divided by 1000 ("Midnight Cort × ACTH/1000") showed an area under the curve (AUC) of 0.902 for identifying EAS, with a cut-off value >2 (sensitivity 83.3%, specificity 84.3%). Its diagnostic accuracy was comparable to that of the high-dose dexamethasone suppression test (HDDST).;EAS patients were older and exhibited more severe metabolic abnormalities than those with CD. Elevated tumor markers may suggest ectopic ACTH secretion. The "Midnight Cort × ACTH/1000" index provides high diagnostic value and a simple, convenient method for differentiating EAS from CD.;
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