What test is primarily used to differentiate Cushing's disease from Cushing's syndrome caused by adrenal tumors?

The serum ACTH test is a critical diagnostic tool for differentiating between Cushing's disease, which is typically caused by a pituitary adenoma secreting adrenocorticotropic hormone (ACTH), and Cushing's syndrome due to adrenal tumors that produce cortisol independently of ACTH. In cases of Cushing's disease, the level of serum ACTH will be elevated or inappropriately normal due to the pituitary secretion. Conversely, when Cushing's syndrome is due to adrenal tumors, ACTH levels are usually suppressed because the high levels of cortisol generated by the tumor inhibit the secretion of ACTH through negative feedback.

In summary, measuring serum ACTH levels helps to identify the source of excess cortisol production. Elevated ACTH suggests Cushing's disease, while low ACTH indicates an adrenal source of cortisol excess, assisting medical professionals in determining the appropriate management and treatment for the patient.

The other options, while useful in the broader context of diagnosing and understanding Cushing's syndrome and disease, do not provide as direct a means to distinguish between the specific etiology of Cushing's disease versus adrenal tumors as the serum ACTH test does.