High dose dexamethasone test: This test is used to distinguish pituitary and adrenal dependant disease in dogs once Cushings has been diagnosed and can help with diagnosis in cats. Dexamethasone dose is 0.1mg/kg (Bexfield and Lee, 2010). Pituitary dependant disease will show suppression of cortisol production due to negative feedback at the pituitary whereas adrenal dependant disease will not.
Ultrasound of the adrenal glands can also be performed to distinguish if one is large or if both are bilaterally enlarged. This would distinguish from PDH (bilateral) and an adrenal tumour (one enlarged).