Pathophysiology-hyponatremia

(JASN 2008;19:1076; NEJM 2015;372:1349)

Pathophysiology

(JASN 2008;19:1076; NEJM 2015;372:1349)

• Excess H2O relative to Na, usually due to ↑ ADH
• ↑ ADH may be appropriate (eg, hypovolemia or hypervolemia with ↓ EAV)
• ↑ ADH may be inappropriate (SIADH)

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• Rarely, ↓ ADH (appropriately suppressed), but kidneys unable to maintain nl [Na]serum
• at steady state, solute intake = solute excretion; urine output = solute excretion/Uosm
• nl dietary solute load ~750 mOsm/d, min Uosm = 50 mOsm/L, ∴ UOP can be up to ~15 L ↑ H2O intake (1° polydipsia): ingestion of massive quantities (usually >15 L/d) of free H2O overwhelms diluting ability of kidney → H2O retention ↓ solute intake (“tea & toast” & beer potomania): ↓↓ daily solute load → insufficient solute to excrete H2O intake (eg, if only 250 mOsm/d, minimum Uosm = 50 mOsm/L → excrete in ~5 L; if H2O ingestion exceeds this amount → H2O retention)