Algorithm for differentiation of Cushing's syndrome. The first step in the differentiation of Cushing's syndrome after diagnosing hypercortisolism is measurement of plasma adrenocorticotropic hormone (ACTH) levels. Typically, these should be reduced after the morning hours (see Fig. 4-4). In pituitary Cushing's disease and ectopic forms of Cushing's syndrome, elevated values are observed, especially in the afternoon and evening. The next step in differentiation is an anatomic evaluation of the pituitary. When no abnormality is found, the next step is a search for a malignancy, typically in the lung. The finding of low ACTH levels points to the adrenal as the source of excessive cortisol production, and anatomic studies of the adrenal are indicated. CT— computed tomography; MRI—magnetic resonance imaging.
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