36 year old female , asymptomatic but with raised 24-hour urinary coisol level and raised ACTH with suppression in high dose dexamethasone suppression test. MRI of head reveals no enlargement of the pituitary gland. What should be the next step in this case?
MRI to assess the adrenals CT chest Inferior petrosal sinus venous sampling Coicotropin-releasing hormone (CRH) levels