Hyponatremia ADH = antidiuretic hormone; Na + = sodium; P osm = plasma osmolality PROFILE Definition Hyponatremia is defined as a plasma sodium (Na + ) concentration < 140 mEq/L in dogs and 150 mEq/L in cats. 1 Because plasma Na + concentration is the main determinant of plasma osmolality (P osm ), hyponatremia usually reflects hypoosmolality. Since the kidneys are highly efficient in excreting water, hypoosmolality almost always implies a defect in renal water excretion and excess of water in

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... he body due to one of the following mechanisms: G Impaired diluting ability in the loop of Henle and distal tubule (volume depletion, renal failure, diuretics) G Increased collecting-tubule permeability due to the presence of antidiuretic hormone (ADH) (effective circulating volume depletion, inappropriate ADH secretion, pain, adrenal insufficiency, or hypothyroidism) Consultant on Call / NAVC Clinician's Brief / February 2010 .