ACTH level: This test helps distinguish an adrenal source (ACTH independent) from an ACTH dependent cause. A very low ACTH level suggests the problem is coming from the adrenal glands, and not the pituitary gland.
High dose Dexamethasone suppression test: This test helps to distinguish patients with excess production of ACTH due to pituitary adenomas from those with ectopic ACTH-producing tumors. Patients are given 2 mg dexamethasone by mouth every 6 hours for 2 days. A 24-hour urine collection for cortisol is performed on the second day of the test. Cortisol suppression suggests a pituitary tumor. A similar test is performed using a single dose of 8 mg at midnight, and a fasting blood draw for cortisol the next morning.
Petrosal sinus sampling: This test may be required to separate pituitary from ectopic causes of ACTH-dependent Cushing's syndrome in patients with a normal pituitary gland on brain MRI scan. Catheters are placed through a vein in the upper thigh/groin region and are threaded through the blood vessels to the petrosal sinuses, which surround the pituitary gland. ACTH levels higher in the petrosal sinuses than in a forearm vein indicate the presence of a pituitary adenoma (Cushing's disease); similar levels suggest ectopic ACTH syndrome.
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