Background <p>Ectopic Cushing syndrome ECS is actually a rare form of endogenous Cushing syndrome, yet it can be life-threatening because of corticotropin-releasing hormone CRH or even adrenocorticotropic hormone ACTH that some non-pituitary tumors then secrete. ECS is often with neuroendocrine tumors such as NETs like SCLCs and bronchial carcinoids and accounts for about 10% of Cushing syndrome cases. ECS poses a diagnostic and therapeutic problem given biochemical intricacy. Its varied presentations in underlying malignancies contribute to this challenge too. ACTH-secreting NETs in lungs, thymus, pancreas, and other organs most frequently cause ECS. These NETs include especially bronchial carcinoids and also SCLCs. Classical features of Cushing syndrome range from atypical features with rapidly progressing severe hypercortisolaemia. Common occurrences include cardiovascular complications, metabolic derangements, and infections, along with psychiatric disturbances. Prognosis is largely determined by tumor type, metastatic spread, severity of cortisol excess, and time to diagnosis. SCLCs and thymic carcinoids are associated with the worst outcomes, while early identification and resection of occult or low-grade NETs can result in favorable long-term survival.</p> Case Report <p>A 36-year-old male presented with severe hypercortisolaemia and signs of Cushing syndrome, with cardiac dysfunction initially misattributed to myocarditis. Biochemical testing and imaging suggested ectopic ACTH secretion from a lung lesion. Initial therapy with ketoconazole was replaced by osilodrostat due to hepatotoxicity. Surgical resection of a typical carcinoid tumour led to biochemical remission and marked improvement in cardiac function. Postoperative follow-up revealed persistent adrenal suppression, rebound thymic hyperplasia, and elevated methoxycatecholamines. The thymic enlargement was histologically confirmed to be benign. Methoxycatecholamine elevation was hypothesized to result from delayed medullary adaptation posthypercortisolaemia.</p> Conclusions <p>ECS can present with multisystem involvement, including reversible cardiomyopathy, endocrine dysfunction, and immunologic rebound phenomena. Prompt biochemical diagnosis, targeted imaging, and cortisol-lowering therapies are critical for patient stabilization before definitive tumour resection. Post-remission, vigilance for complications such as adrenal insufficiency, thymic hyperplasia, and persistent catecholamine alterations is essential.</p>