Abstract Disclosure: D. Mendez: None. M.G. Fernandez: None. D.J. Selen: None. Background: Endogenous Cushing syndrome may result from an ectopic non-pituitary tumor. Severe hypercortisolism has high mortality, so prompt medical management to acutely decrease cortisol levels is necessary when no surgical options are available. Clinical Case: A 67-year-old male presented to the hospital with four weeks of fatigue and was found to have a right hilar mass with unsuppressed cortisol levels for which an ectopic tumor was suspected. On initial evaluation, the patient presented with hypokalemia, acute kidney injury, and metabolic alkalosis. In addition, a CT of the chest and abdomen with contrast revealed a right hilar mass with multiple liver lesions concerning for metastasis. These findings prompted workup for hypercortisolism, including 1mg and 8 mg dexamethasone suppression tests. Both tests demonstrated unsuppressed cortisol levels above 150 (reference range [RR] 5.5-20 ug/dl and assay maximum>150) and elevated 24-hour urinary cortisol of 8125 (RR <60 ug/dl), suspicious for an ACTH-secreting tumor. Subsequently, he underwent a liver biopsy to characterize the tumor further and was started on Bactrim for Pneumocystis pneumonia prophylaxis. Metyrapone and osilodrostat were considered for treatment but not available at our facility such that treatment was initiated with ketoconazole 200 mg orally twice daily with subsequent dose titration. He had a mild elevation of AST 52 (RR 7-42 IU/L) and ALT 52 (RR 10-45); therefore, his liver profile was monitored daily due to his elevated risk for hepatotoxicity. With the initiation of ketoconazole, cortisol serum levels decreased to 145 ug/dl after two days of treatment. The pathology report revealed small lung cell carcinoma as the etiology for ectopic Cushing’s syndrome, for which the oncology team-initiated etoposide and carboplatin. Unfortunately, on day 6 of the initiation of treatment, the patient rapidly deteriorated and passed away from pneumonia and septic shock. Conclusion: Severe hypercortisolism leads to an immunosuppressed state resulting in high mortality. Therapeutic options depend significantly on the accessibility of first-line treatment options and prompt initiation of medical treatment. When no other agents are available, ketoconazole should be started at high doses despite the mild elevation of liver enzymes with close monitoring of LFTs. While it is unclear if the patient’s deterioration was due to his cancer burden, chemotherapy, severe hypercortisolism, or a combination, given that osilodrostat and metyrapone are first-line treatment options, standardized systems should be reviewed in hospitals for faster acquisition of these medications to avoid delays in treatment initiation. In addition, the decision to transfer patients to the intensive care unit for etomidate infusion while awaiting the acquisition of the above medications should be considered promptly. Presentation: Friday, June 16, 2023