What are the possible causes of polyuria/polydipsia in dogs and cats?

The inability to concentrate urine due to chronic loss of sodium, solutes in the renal medulla, and hypertonic medullary gradient is a possible explanation for polyuria/polydipsia in animals with Addison's disease.
Primary diabetes insipidus is an extremely rare congenital defect characterized by a lack of affinity and response of the V2 receptors of the distal and collecting tubules to ADH action.
Infections and inflammation of the renal pelvis can alter the countercurrent mechanism in the renal medulla, leading to isosthenuria, secondary polyuria, and compensatory polydipsia.
The presence of leucocytosis is frequently observed in pyometra and pyelonephritis.
The presence of symmetric alopecia, bulging belly, muscle weakness, and hepatomegaly suggests hyperadrenocorticism.