In a patient with suspected Cushing's syndrome, what would a responsive test indicate?

In the context of suspected Cushing's syndrome, a responsive test, typically involving the administration of a low-dose dexamethasone suppression test, is utilized to differentiate between different sources of cortisol production and ACTH secretion. A responsive result from this test would indicate that the source of ACTH is likely from the pituitary gland, also known as Cushing's disease.

In this scenario, the mechanism is based on the feedback inhibition of cortisol on ACTH production. If the pituitary is secreting ACTH in response to the cortisols, then administration of dexamethasone (which mimics cortisol) would lead to reduced ACTH secretion due to negative feedback. This responsiveness confirms that the pituitary gland is the probable source of the increased ACTH levels seen in Cushing's syndrome.

On the other hand, if there is an adrenal tumor (which would produce cortisol independent of ACTH) or ectopic production of ACTH (from a tumor elsewhere in the body), the negative feedback would not result in a suppression of ACTH, as these would not be responsive to dexamethasone in the same way. Primary adrenal insufficiency does not apply here as it refers to decreased cortisol production due to adrenal gland failure