SIADH

The patient's sodium is 118 mEq/L but they're not dehydrated — they're drowning from the inside. SIADH flips everything you'd expect about fluid balance.

Syndrome of Inappropriate Antidiuretic Hormone (SIADH) occurs when the posterior pituitary releases excess ADH despite normal or low serum osmolality, causing the kidneys to reabsorb too much water. The result is dilutional hyponatremia — the body has too much water, not too little sodium. Serum sodium drops below 135 mEq/L (often critically below 120), serum osmolality falls below 275 mOsm/kg, while urine becomes inappropriately concentrated (urine specific gravity >1.030, urine osmolality elevated). The patient gains water weight without visible edema because the excess fluid is intracellular. Common triggers include lung cancers (especially small cell), head injuries, CNS infections, and medications like SSRIs, carbamazepine, and cyclophosphamide. Priority nursing management centers on fluid restriction (typically 500–1000 mL/day), strict I&O, daily weights, and monitoring neurological status. Severe hyponatremia (<120 mEq/L) causes seizures, coma, and death. Hypertonic saline (3% NaCl) may be ordered for critical cases but must be infused slowly — correcting sodium faster than 8–12 mEq/L per 24 hours risks osmotic demyelination syndrome. Demeclocycline may be used to block ADH action on the kidneys.

Key Distinctions

Don't confuse SIADH with diabetes insipidus — SIADH is too much ADH (water retention, dilutional hyponatremia, concentrated urine), while DI is too little ADH (massive water loss, hypernatremia, dilute urine). They are mirror opposites. Students often think SIADH causes edema — it doesn't, because the excess water distributes intracellularly and the body compensates by suppressing aldosterone, so peripheral edema is absent.

Clinical Pearl

SIADH = Swimming In A Diluted Hole. Too much water in, sodium diluted down, urine concentrated up. Restrict fluids, not sodium.