Sodium in Dogs: What High and Low Na+ Levels Mean

Sodium is the electrolyte the body uses to manage water. Where sodium goes, water follows — so a sodium value that's off tells you something important about how the body is handling fluid. Low sodium usually means too much water relative to sodium (dilutional) or direct sodium loss; high sodium usually means too little water relative to sodium. The causes are very different and some require urgent action.

Normal Sodium Range in Dogs

Normal sodium (Na+) in dogs is 140–155 mEq/L. Sodium is the principal cation (positively charged ion) in extracellular fluid — it determines plasma osmolality and governs the distribution of water between the intracellular and extracellular compartments. Even a 10 mEq/L deviation from normal has clinical significance.

Low Sodium (Hyponatremia) in Dogs

Hypoadrenocorticism (Addison's Disease)

Addison's disease is the most important cause of low sodium in dogs and must be considered whenever Na+ falls below the reference range — especially alongside elevated potassium. Aldosterone, the adrenal hormone that retains sodium in the kidney tubule, fails. Sodium is lost in the urine while potassium accumulates. The resulting low Na + high K combination, and particularly the Na:K ratio below 27, is the classic Addisonian electrolyte signature. These dogs are typically weak, sometimes collapsed, often bradycardic. The ACTH stimulation test confirms the diagnosis.

Syndrome of Inappropriate ADH (SIADH)

ADH (antidiuretic hormone) tells the kidneys to retain water. In SIADH, ADH is secreted inappropriately — often from a brain lesion, thoracic mass, or drug effect — causing the body to hold onto more water than it needs. The sodium becomes diluted. The key differentiator from other causes: urine sodium is paradoxically concentrated (the kidneys are holding onto sodium but still retaining even more water). SIADH in dogs is less common than in humans but documented.

Congestive Heart Failure (CHF)

Dogs with advanced heart failure retain fluid through multiple compensatory mechanisms (aldosterone activation, ADH release, reduced cardiac output). This retained fluid dilutes sodium. The hyponatremia here is dilutional — total body sodium may actually be high, but the concentration drops because water accumulates faster. This pattern is associated with a worse prognosis in heart failure dogs.

Liver Failure and Cirrhosis

Portal hypertension from liver disease causes fluid to accumulate in the abdomen (ascites) and activates the same compensatory fluid-retaining hormones as heart failure. Hyponatremia follows from the same dilutional mechanism. Dogs with cirrhosis, portosystemic shunts, or chronic hepatitis may all develop this picture.

Psychogenic Polydipsia

Some dogs — particularly anxious breeds or dogs with behavioral issues — drink compulsively. Excess water consumption dilutes plasma sodium. The urine is typically very dilute (specific gravity < 1.006). This is a diagnosis of exclusion after ruling out diabetes insipidus and other causes.

GI Loss Replaced with Water

A dog that loses large amounts of sodium through vomiting or diarrhea, then drinks water to compensate, can develop hyponatremia — water replaces the volume but not the electrolytes. This is self-correcting once eating and drinking normally resume, but can be significant with prolonged illness.

High Sodium (Hypernatremia) in Dogs

Dehydration

The most common cause of mild hypernatremia is simple dehydration — the dog has lost water through panting, inadequate intake, vomiting, or diarrhea without replacement. Sodium concentration rises as water volume falls. This is usually accompanied by other signs of dehydration: elevated PCV/HCT, elevated total protein, and elevated BUN:creatinine ratio. Rehydration corrects the sodium.

Diabetes Insipidus (DI)

Diabetes insipidus is a disorder of water balance, not glucose — despite the name. In central DI, the brain doesn't produce enough ADH. In nephrogenic DI, the kidneys don't respond to ADH. The result is massive production of dilute urine regardless of how concentrated the blood becomes. Dogs with DI drink enormous volumes of water (polydipsia) and urinate constantly (polyuria) — but still develop hypernatremia when fluid intake can't keep pace. DI is confirmed by measuring urine specific gravity during water deprivation, or by response to DDAVP (synthetic ADH).