Dual bronchial carcinoids and Cushing's syndrome with a paradoxical response to dexamethasone and a false positive outcome of inferior petrosal sinus sampling.

Objective Establishing the cause of Cushing's syndrome can be a considerable challenge, in particular in ectopic ACTH syndrome, and often requires a combination of biochemical tests and imaging procedures. Subject A 27-year-old man presented with signs of Cushing's syndrome. P-ACTH levels were 3 times above the ULN and free urinary cortisol in the order of 1900 nmol/24 h. The work-up showed remarkable results. Results A 2-day low-dose dexamethasone suppression test demonstrated paradoxical increases in cortisol. Sampling from the inferior petrosal sinuses (BIPSS) showed a central to peripheral ACTH ratio of 4.7 after CRH stimulation, i.e. indicated pituitary disease, but MRI of the pituitary was normal and baseline P-CRH <1.0 (ref.