Etiology	Description	Evaluation
Increased free water loss
Diabetes insipidus	May be due to central diabetes insipidus (insufficient production of antidiuretic hormone [arginine vasopressin]) or nephrogenic diabetes insipidus (decreased renal responsiveness to arginine vasopressin) Low activity causes the excretion of inappropriately dilute urine Patients may compensate by increasing fluid intake, resulting in polydipsia and polyuria; this increased intake of free water may attenuate the increase in sodium	Serum and urine electrolytes showing hypotonic urine Water deprivation testing can confirm diagnosis, and arginine vasopressin (desmopressin) stimulation test can determine central vs. nephrogenic diabetes insipidus
Environmental exposure	Prolonged exposure to hot, arid environments	Clinical history
Gastrointestinal losses	Fluid loss from vomiting or diarrhea; nausea may prevent patients from consuming enough fluid	History of nausea, vomiting, or diarrhea
Iatrogenic	Overdiuresis	History of diuretic use
Increased metabolic demands	Fever or increased physical activity	Clinical history
Sweating	Fluid lost through sweat, such as from environmental exposure or exercise	Clinical history
Decreased free water intake
Hypodipsia	Patients who are physically and cognitively capable of obtaining free water yet fail to do so; may be due to decreased thirst response, which can occur in older adults or people with psychiatric disorders such as depression	Clinical history
Iatrogenic	Patients who are restrained, sedated, intubated, not allowed to drink, or housed in a long-term care facility may not receive adequate amounts of water from medical personnel	Clinical history
Physical and cognitive limitations	Patients who are reliant on others to provide sufficient water or those who are unable to obtain safe water; may include young children, patients with disabilities, or those with poor access to water due to disaster, falls, or other adverse circumstances	Clinical history
Increased sodium intake
Iatrogenic	Hypertonic dialysis or administration of fluids with a supraphysiologic salt load such as hypertonic saline or sodium bicarbonate	History of dialysis or medication administration
Salt toxicity	Rapid ingestion of excessive amounts of salt can acutely increase the serum sodium concentration A rare condition with high mortality	History of excessive salt ingestion