The constellation of worsening glycemic control, hypertension, hypokalemia, and weight gain strongly supports the diagnosis of Cushing syndrome. In this case, the elevated ACTH level did not indicate a cortisol-producing adrenal tumor, which would suppress ACTH. The patient had normal head MRI findings, which does not exclude pituitary Cushing syndrome because these tumors are often small. The nonsuppressible high-dose dexamethasone suppression test results rule out pituitary Cushing syndrome, which would suppress with high doses of dexamethasone but not with low doses. Some bronchial carcinoids produce corticotropin-releasing hormone (CRH) instead of ACTH and otherwise mimic pituitary Cushing syndrome. Even inferior petrosal sinus sampling (IPSS) for ACTH suggests pituitary Cushing syndrome in these cases, and bronchoscopic evaluation may be required.
A low-risk procedure such as bronchoscopy or biopsy is available in most centers and should be performed first if the index of suspicion is high for a bronchial carcinoid.
In this patient, the surgical excision of carcinoid tumor was accompanied by rapid normalization of blood pressure and hypokalemia. Plasma ACTH and cortisol levels rapidly decreased over the next few days. He was sent home on prednisone, which was tapered over 12 weeks to 5 mg/day and completely stopped at 6 months, when the hypothalamic-pituitary-adrenal axis became fully operational. By then, he had lost 30 lb. His fasting blood glucose level was 112 mg/dL, and insulin was stopped. His glycated hemoglobin was 7.3%.
Association between nonpituitary or adrenal neoplasm and Cushing syndrome dates back to 1928 when Brown first described a case with small cell lung cancer with features resembling Cushing syndrome. ACTH was later shown to be produced by these neoplasms. The spectrum of ACTH-secreting neoplasms now extends to carcinoid tumors, pheochromocytomas, medullary thyroid carcinoma, and islet cell tumors. Other rare ectopic locations have been described as well. Occasionally the driver may be ectopic CRH.
Ectopic ACTH/CRH secretion accounts for 10% to 15% of Cushing syndrome cases. Bronchial carcinoid tumors represent 1% to 5% of all lung tumors; approximately 1% to 5% of them are associated with ectopic ACTH secretion. Bronchopulmonary tumors are considered to be l-w grade, although some can metastasize. They have epithelial and neuroendocrine differentiation. Resolution of hormonal abnormalities is quite rapid. Somatostatin scintigraphy and selective pulmonary artery sampling for ACTH have been used to localize carcinoid tumors.
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