Abstract Disclosure: K.A. Arao: None. A.E. Gadbois: None. Introduction: Adult idiopathic ACTH deficiency (AIAD) is a rare and potentially life-threatening condition that is frequently misdiagnosed or even missed. We present a case of AIAD in a patient who exhibited altered mental status alongside a COVID-19 infection and was fortunately diagnosed after responding to empiric treatment with oral steroids for another condition. Case Presentation: A 76-year-old man was admitted to the hospital for bilateral leg weakness. His hospital stay was complicated by the development of febrile episodes and delirium. He tested positive for COVID-19, and the rest of infectious work-up yielded negative results. He then received a course of remdesivir. In the evaluation of his delirium, a CT scan of the brain, MRI of the brain, and lumbar puncture with CSF analysis were done, all of which were unremarkable. Initially, the delirium was attributed solely to COVID-19 infection. After consultation with neurology and rheumatology, the patient received empiric treatment with prednisone 5 mg twice daily for possible polymyalgia rheumatica as a cause of his bilateral leg weakness. His mental status significantly improved following steroid administration. With the improvement in symptoms, an AM cortisol test was added on his morning blood tests, revealing a cortisol level of 2.56 mcg/dL with an albumin level of 2.3 g/dL, raising concerns for adrenal insufficiency. An outpatient endocrinology evaluation recommended transitioning from prednisone to physiologic twice-daily dosing of hydrocortisone. An ACTH stimulation test was performed, which peaked at an AM cortisol of 6.1 mcg/dL, with a baseline ACTH level of 4.1 pg/mL. A review of the recent brain MRI showed no pituitary pathology. Other pituitary hormones were normal, and he was eventually diagnosed with AIAD. He was subsequently maintained on physiologic dosing of hydrocortisone, resulting in clinical stability. Discussion: AIAD is characterized by secondary adrenal insufficiency, normal production of other pituitary hormones, and the absence of structural pituitary pathology. The etiology of AIAD remains unknown, with most cases attributed to an autoimmune process. AIAD can have diverse clinical manifestations and may only become more apparent with triggers like infections or illnesses. As in our case, the COVID-19 infection likely unmasked the underlying AIAD, presenting with altered mental status. Identifying AIAD can be challenging, leading to high rates of misdiagnosis and clinical omission. Physicians should consider AIAD when dealing with unexplained altered mental status, as the administration of a simple treatment with steroids can be life-changing. Presentation: 6/3/2024