What causes polyuria?

Causes of Polyuria

Polyuria results from either excessive solute excretion (osmotic diuresis) or impaired urinary concentration (water diuresis), with the most common causes being diabetes mellitus, diabetes insipidus (central or nephrogenic), primary polydipsia, chronic kidney disease, and medication effects. 1, 2

Classification by Mechanism

Polyuria is pathophysiologically divided into two main categories based on urine osmolality 1:

Osmotic Diuresis (Urine Osmolality >300 mOsm/L)

• Diabetes mellitus is a leading cause, with hyperglycemia producing classic symptoms of polyuria, polydipsia, and unexplained weight loss 3
• Chronic kidney disease with impaired concentrating ability causes solute diuresis 2
• Electrolyte disorders, particularly hypokalemia (as seen in aldosteronoma), can inhibit insulin secretion and cause polyuria 3

Water Diuresis (Urine Osmolality <150 mOsm/L)

• Central diabetes insipidus (AVP deficiency) results from deficient vasopressin production with low or absent plasma ADH levels 4, 5
• Nephrogenic diabetes insipidus (AVP resistance) occurs when the distal nephron is insensitive to ADH despite normal or elevated plasma ADH levels 4, 6
• Primary polydipsia presents with excessive fluid intake and appropriately dilute urine 7, 8

Endocrine Causes

• Acromegaly, Cushing's syndrome, glucagonoma, and pheochromocytoma produce hormones that antagonize insulin action, causing hyperglycemia and polyuria in individuals with preexisting insulin secretion defects 3
• Hyperthyroidism or profound hypothyroidism can contribute to polyuria 3
• Testosterone deficiency may be associated with polyuria 3

Renal Tubular Disorders

• Bartter syndrome presents with polyuria, particularly types 1,2, and 4, often with severe polyhydramnios prenatally due to excessive fetal polyuria 3
• Diseases of the exocrine pancreas (pancreatitis, cystic fibrosis, hemochromatosis) damage β-cells when extensive enough, causing diabetes and subsequent polyuria 3

Medication-Induced Polyuria

• Diuretics are an obvious cause of increased urine output 3
• Lithium commonly causes nephrogenic diabetes insipidus 3
• Calcium channel blockers can contribute to polyuria 3
• NSAIDs may affect renal concentrating ability 3
• Glucocorticoids can impair insulin secretion and cause drug-induced diabetes 3

Sleep-Related Causes

• Obstructive sleep apnea is associated with nocturia and nocturnal polyuria, with some patients becoming dry after upper airway obstruction is relieved 3

Cardiovascular and Renal Causes

• Congestive heart failure causes nocturnal polyuria due to mobilization of peripheral edema during recumbency 3
• Hypertension may be associated with nocturia and polyuria 3

Critical Diagnostic Distinction

The key to diagnosis is measuring urine osmolality alongside serum osmolality 1, 8:

• Inappropriately dilute urine (osmolality <300 mOsm/kg) with normal-to-high serum osmolality is pathognomonic for diabetes insipidus 6
• Low serum osmolality with dilute urine suggests primary polydipsia 8
• High urine osmolality indicates solute diuresis from diabetes mellitus, chronic kidney disease, or normal physiologic response 8

High-Risk Populations

• Infants are particularly vulnerable as they lack free access to fluid and cannot communicate thirst, with congenital nephrogenic diabetes insipidus typically diagnosed around 4 months of age 9
• Adults with impaired thirst or restricted mobility (neurological impairment) face increased dehydration risk from polyuria 9
• Children with type 1 diabetes typically present with hallmark polyuria/polydipsia symptoms 3