What are the expected laboratory findings in primary polydipsia (Psychogenic Polydipsia)?

From the Guidelines

In psychogenic polydipsia, expected laboratory findings include dilutional hyponatremia, low urine osmolality, and decreased serum osmolality, with appropriately suppressed antidiuretic hormone (ADH) levels. The diagnosis of psychogenic polydipsia is often considered in the context of differential diagnoses for nephrogenic diabetes insipidus (NDI), as seen in a recent international expert consensus statement 1. Key laboratory features of psychogenic polydipsia include:

• Serum sodium <135 mEq/L due to excessive water intake
• Urine osmolality <100 mOsm/kg, reflecting the dilutional state
• Serum osmolality <280 mOsm/kg, indicating decreased serum osmolality
• Inappropriately normal or elevated urine sodium levels despite hyponatremia
• Normal or low blood urea nitrogen (BUN) and creatinine, unless renal function is compromised
• Decreased hemoglobin and hematocrit due to hemodilution, as seen on complete blood count These laboratory findings are consistent with the pathophysiology of excessive water intake overwhelming normal regulatory mechanisms, leading to dilution of electrolytes and decreased serum osmolality without primary renal or endocrine dysfunction, as distinguished from other conditions such as acquired NDI, central DI, or primary polydipsia 1.

From the Research

Expected Labs in Psychogenic Polydipsia

• Sodium levels:
  • Typically low, ranging from 98 to 124 mEq/L 2
  • Can be as low as 108 mEq/L 3
• Plasma osmolality:
  • Ranges from 236 to 244 mOsm/kg 2
• Urine osmolality:
  • Can be dilute, ranging from 37 to 95 mOsm/kg during water load 2
  • Exceeds plasma osmolality when plasma concentration rises to between 242 and 272 mOsm/kg during fluid deprivation 2
• Free water clearance:
  • Ranges from 12 to 36 L/day 2

Other Laboratory Findings

• Metabolic encephalopathy secondary to severe hyponatremia may be present 3
• Laboratory evidence for rhabdomyolysis may be seen in some patients 4