From the Research
To differentiate between SIADH and psychogenic polydipsia, focus on urine and serum osmolality patterns, with the most recent and highest quality study suggesting that urine osmolality and sodium levels are key diagnostic factors 1. When evaluating urine studies, consider the following key points:
- In SIADH, urine osmolality is typically high (>100 mOsm/kg, often >300 mOsm/kg) despite hyponatremia and low serum osmolality (<280 mOsm/kg) 2.
- Urine sodium is usually elevated (>40 mEq/L) in SIADH, reflecting the body's attempt to retain sodium in the face of excessive ADH secretion 2.
- In contrast, psychogenic polydipsia presents with dilute urine (urine osmolality <100 mOsm/kg) and low urine sodium (<20 mEq/L) as the kidneys attempt to compensate for excessive water intake 3. A water restriction test can be diagnostic, as in SIADH, serum sodium fails to normalize with water restriction, while in psychogenic polydipsia, serum sodium normalizes and urine concentrates appropriately 1. Other supporting findings for SIADH include euvolemia on physical exam, normal renal, adrenal and thyroid function, and absence of diuretic use 3. For accurate results, collect urine samples before fluid restriction or treatment is initiated, as these interventions can alter the natural osmolality patterns 2. The distinction between SIADH and psychogenic polydipsia is crucial, as treatment approaches differ significantly - fluid restriction for SIADH versus behavioral therapy and monitored fluid intake for psychogenic polydipsia 1.