Differential Diagnosis of Hyperkalemia and Hyponatremia in Dogs and Cats

Hyperkalemia and hyponatremia are the classic electrolyte abnormalities found in dogs and cats with naturally occurring primary hypoadrenocorticism (Addison’s disease) and are seen in more than 80% of affected animals. In addition, a significant number of dogs with spontaneous secondary hypoadrenocorticism (isolated adrenocorticotropic hormone [ACTH] deficiency) are hyponatremic. Although relatively uncommon, hypoadrenocorticism is often the first disease that practitioners consider when these serum electrolyte abnormalities are detected. These electrolyte disturbances alone, however, are not sufficient for such a diagnosis. Definitive diagnosis of hypoadrenocorticism requires demonstration of inadequate adrenal reserve in response to exogenous ACTH administration. The ACTH stimulation test is considered the “gold standard” for the diagnosis of hypoadrenocorticism in both dogs and cats. Dogs with hypoadrenocorticism have a low to undetectable resting serum cortisol concentration, and the response to exogenous ACTH administration is diminished or absent (post-ACTH plasma cortisol concentration <2 µg/dl or <55 nmol/L). In dogs without hypoadrenocorticism (nonadrenal illness), the resting serum cortisol concentration may also be low if they have been treated with one or more doses of glucocorticoids (secondary to feedback inhibition of ACTH), but normal adrenocortical responsiveness is maintained (post-ACTH plasma cortisol concentration >5 to 7 µg/dl or >140 to 195 nmol/L).

This chapter reviews the differential diagnosis of both hyperkalemia and hyponatremia in dogs and cats that have normal cortisol responses after ACTH stimulation. In general, a wide variety of diseases more prevalent than hypoadrenocorticism may be associated with hyperkalemia and hyponatremia, such as renal disease, gastrointestinal disorders, heart failure, and acidosis.

Differential Diagnosis

Various disorders in dogs and cats are associated with hyperkalemia or hyponatremia (Web Box 16-1). Although some of these disorders are not commonly recognized by the practicing veterinarian as causes of hyperkalemia or hyponatremia, they must be considered for the clinician to arrive at the correct diagnosis. The following compilation is not comprehensive but does reflect clinically relevant situations. The reader is referred to Rose (1989) and DiBartola (2011) for complete reviews of disorders of sodium and potassium.

Web Box 16-1 Common Causes of Hyperkalemia and Hyponatremia

Hyperkalemia