In ectopic ACTH-secreting pheochromocytoma, combined ACTH-driven hypercortisolemia and hypercatecholaminemia are serious conditions, which can be fatal if not diagnosed and managed appropriately, especially when glucocorticoid-driven positive feedback is suggested with a high ACTH/cortisol ratio. A 46-year-old man presented with headache, rapid weight loss, hyperhidrosis, severe hypertension and hyperglycemia without typical Cushingoid appearance. Endocrinological examinations demonstrated elevated plasma and urine catecholamines, serum cortisol and plasma ACTH. Moreover, his ACTH/cortisol ratio and catecholamine levels were extremely high, suggesting catecholamine-dominant ACTH-secreting pheochromocytoma. Computed tomography revealed a large right adrenal tumor.18F-FDG positron emission tomography showed uptake in the area of the adrenal tumor, while123I-metaiodobenzylguanidine scintigraphy showed no accumulation. His plasma ACTH level paradoxically became elevated after a dexamethasone suppression test. After metyrapone administration, not only serum cortisol but also plasma ACTH levels were exponentially decreased almost in parallel, suggesting a glucocorticoid-driven positive-feedback regulation in this rapidly exacerbated ectopic ACTH-producing pheochromocytoma. Interestingly enough, plasma catecholamine levels were also decreased by metyrapone, although they remained extremely high. He became severely dehydrated due to hypoadrenalism requiring hydrocortisone supplementation. His clinical signs and symptoms were improved, and right adrenalectomy was performed uneventfully, resulting in complete remission of pheochromocytoma and Cushing’s syndrome. A glucocorticoid-driven positive-feedback regulation in this ectopic ACTH-secreting pheochromocytoma created a vicious cycle with rapid exacerbation of both hypercortisolemia and hypercatecholaminemia with extremely elevated plasma ACTH level. Metyrapone was clinically effective to stop this vicious cycle; nonetheless, great care must be taken to avoid hypoadrenalism especially when hypercatecholaminemia remained.
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