What is the most appropriate next diagnostic step for a dog with increased thirst, urination, and skin thinning?

The most appropriate next diagnostic step for a dog presenting with increased thirst, urination, and skin thinning is low-dose dexamethasone suppression testing. This test is specifically designed to assess for Cushing's disease (hyperadrenocorticism), which is characterized by excessive cortisol production. The clinical signs described—polydipsia (increased thirst), polyuria (increased urination), and skin thinning—are highly suggestive of this condition.

During low-dose dexamethasone suppression testing, a small dose of dexamethasone, a synthetic glucocorticoid, is administered to evaluate the responsiveness of the adrenal glands to feedback regulation. In healthy animals, the administration of dexamethasone would suppress cortisol production due to negative feedback mechanisms. In cases of Cushing's disease, cortisol production is not adequately suppressed, thus indicating the presence of the condition.

Other diagnostic steps, such as measuring baseline cortisol concentration, could provide initial information regarding cortisol levels but would not confirm Cushing's disease definitively. Abdominal ultrasound may be useful in identifying adrenal tumors or ruling out other abdominal conditions but is not the first-line test to evaluate for hyperadrenocorticism itself. Similarly, measuring endogenous ACTH