Abstract Disclosure: R.C. Attah: None. I. Sirisena: None. Pheochromocytoma/Paragangliomas are rare neurogenic tumors that arise from chromaffin cells present in the adrenal medulla or extra-adrenal paraganglia, these cells produce catecholamines. Typically, these tumors are discovered as incidentalomas. They can be hereditary as seen in genetic syndromes like multiple endocrine neoplasia type 2A or 2B, neurofibromatosis type 1 or Von Hippel Lindau syndrome but can occur sporadically. Patients typically present with hormonal hypersecretion signs or symptoms like abdominal pain, episodic headaches, sweating, hypertension and tachycardia, but some patients are asymptomatic. Patient is a 37-year-old female with past medical history of hypertension, type 2 diabetes mellitus and fatty liver who presented to the ED with abdominal pain for a few weeks. On arrival, her vitals were BP 205/124, HR 102, RR 14 and SpO2 100% on room air. CT abdomen was obtained which showed a 7.6 x 4.7 x 5.7 cm mass replacing left adrenal gland with small left retroperitoneal hemorrhage worrisome for adrenal cortical carcinoma, metastatic disease or pheochromocytoma. Hormonal workup revealed elevated 24-hr urine cortisol 124.2mcg (ref 4.0-50.0mcg/24h), normal aldosterone/renin ratio 10.63ng/dL (ref <30ng/dl), failed low dose dexamethasone suppression test with post-dex AM cortisol 7.20ug/dL (ref <1.8ug/dL), slightly elevated serum total catecholamine 1256pg/mL (242-1125 pg/mL), norepinephrine 1155pg/mL (ref 217-1109 pg/mL), total metanephrine and normetanephrine 341pg/mL (reg <205 pg/mL), normetanephrine 244.1 (ref <210.1pg/mL), elevated 24-hr urine normetanephrine 700ug/L (ref 131-612ug/L), metanephrine 136ug/L (ref 36-209ug/L) and DHEA-s level 285ug/dl (ref 23-266ug/dL). Due to elevation in cortisol, adrenal cortical carcinoma was suspected as these tumors are frequently associated with cortisol secretion. However, pheochromocytoma with co-secretion of ACTH could not be excluded. She was started on doxazosin for alpha blockade and eventually started on beta blockade with Metoprolol. She underwent a left adrenalectomy and was started on steroid taper post-op due to concern of cortisol co-secretion. Surgical pathology was positive for Left adrenal malignant pheochromocytoma and ganglioneuroma. Genetic panel was negative with no mutation identified in the 54 gene Hereditary paraganglioma-pheochromocytoma panel. This patient had a large adrenal tumor (>4cm mass) but mildly elevated catecholamines which could have led to misdiagnosis as her catecholamine levels were less than 2-3x ULN which is unusual for pheochromocytomas. Roughly 25% of people with pheochromocytoma are undiagnosed throughout their lifespan. This condition can be life-threatening, hence early diagnosis and intervention can be lifesaving. Prognosis of malignant pheochromocytoma is dependent on tumor size, presence of SDHB gene mutation or visceral metastases. Presentation: 6/1/2024