Section: Recommended Additional Testing
Tests used to differentiate pituitary- and adrenal-dependent hyperadrenocorticism include:
High dose dexamethasone suppression test. This works on the same principle as the low dose dexamethasone suppression test, but a higher dose of dexamethasone is used. The higher dose usually causes suppression of blood cortisol concentration in dogs with pituitary-dependent hyperadrenocorticism but not in those with adrenal-dependent disease.
Blood ACTH concentration. This test reliably differentiates pituitary- and adrenal-dependent disease, but it is technically difficult to perform. The blood sample must be handled very carefully and sent to a special laboratory to insure accurate results. A low blood concentration of ACTH indicates the presence of an adrenal tumor whereas a normal or high blood concentration of ACTH suggests a pituitary tumor.
Occasionally, the low dose dexamethasone suppression test can differentiate pituitary- and adrenal-dependent causes of Cushing's syndrome. When adequate suppression is observed in the 4-hour sample, but not in the 8-hour sample ("escape"), pituitary-dependent disease should be suspected. Patients with adrenal-dependent disease (adrenal tumors) do not "escape" after suppression.
Presence of one large and one small or normal-sized adrenal gland on abdominal ultrasonography suggests adrenal-dependent disease whereas presence of two large adrenal glands suggests pituitary-dependent disease.
If the tests described above do not allow clear differentiation of pituitary- and adrenal-dependent disease, CT (computerized tomography) or MRI (magnetic resonance imaging) scanning of the brain to look for a pituitary tumor or abdomen to look for an adrenal tumor may be required. These specialized tests require referral to a specialty center.
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