Clinical meaning
SIADH: Ectopic or inappropriate AVP release leads to insertion of Aquaporin-2 channels in the collecting duct, causing pure water retention (Euvolemic Hyponatremia). DI: Lack of AVP (Central) or renal resistance (Nephrogenic) prevents water reabsorption, leading to massive dilute urine output.
Diagnosis & workup
Diagnostics & workup: - Order serum osmolality to confirm hypotonicity (SIADH) or hypertonicity (DI) - Order urine osmolality and urine sodium simultaneously with serum studies - Order serum sodium every 4-6 hours during active correction - Order water deprivation test to differentiate central from nephrogenic DI - Order DDAVP stimulation test (urine concentrates in central DI, not nephrogenic) - Order cortisol and TSH to rule out adrenal insufficiency and hypothyroidism - Order chest imaging and CT head for SIADH etiology workup - Order copeptin level (emerging biomarker for AVP activity)
Risk factors: - Small cell lung cancer (SIADH: ectopic ADH) - Traumatic brain injury (central DI) - Pituitary surgery (transient or permanent DI) - CNS infections (meningitis, encephalitis) - Medications: SSRIs, carbamazepine, cyclophosphamide (SIADH) - Lithium therapy (nephrogenic DI) - Guillain-Barré syndrome - Positive pressure ventilation