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Medical AI Inquiry: You were designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.
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Which of the following tests is best for differentiating ACTH secreting pituitary tumour from Ectopic ACTH production?
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The correct answer is  24 hour urinary coisol levels are elevated in both subsets: ACTH secreting pituitary tumour and Ectopic ACTH secretion. Overnight Dexamethasone suppression test involves overnight 1mg dexamethasone tablet and measurement of 8am morning serum or urinary coisol. It is used to diagnose hypercoisolism but cannot identify etiology of Cushing syndrome. High dose dexamethasone suppression test involves giving 6 hourly dexamethasone 2mg, which leads to etiological diagnosis.( differentiates between ACTH secreting pituitary tumor from Ectopic ACTH ) Serum Coisol> 5mg/dl: Ectopic ACTH production.