Cushing’s disease must be clearly diagnosed in order to avoid unnecessary surgery and poor treatment outcomes. However, because hypercortisolism may arise from exogenous disease that is not a pituitary tumor (most commonly a pulmonary mass), and because pituitary tumors are generally small and difficult to distinguish on imaging, it makes diagnosis difficult.
Confirmation of hypercortisolism: 24-hour urine cortisol tests, daytime plasma cortisol levels, and nighttime salivary cortisol levels are used to detect disturbances in the rhythm of cortisol secretion. Or the dexamethasone suppression test at bedtime.
For differentiating between ACTH-dependent or non-ACTH-dependent Cushing’s disease 1. dexamethasone suppression test, which has to be performed for 6 days to indicate the problem; 2. CRH stimulation test.
3, If both tests are negative and there is no imaging support, but pituitary tumors cannot be ruled out. This is also the time when the subxiphoid sinus sampling test can be used.
It is worth noting that: imaging, even if there is a tumor, does not necessarily cause hypercortisolism by it. This is because a certain percentage of pituitary tumors are present on autopsy and in normal subjects.
Imaging recently has a technique called (SPGR) that can achieve MRI resolution of 1 mm. The traditional resolution of pituitary microadenomas (50-75%) has been improved to at least 80%. However misdiagnosis is still very high 4%.