SIADH vs Diabetes Insipidus on the NCLEX
The classic water-balance comparison: labs, urine, and treatment of each.
Two ADH disorders, exact mirror images. Learn one column and flip the signs for the other.
SIADH: too much ADH
The body retains water. Everything serum is diluted; everything urine is concentrated. Serum sodium LOW (<135), serum osmolality low. Urine concentrated: high specific gravity, low volume. Weight gain without edema (the water is retained, not third-spaced). Danger: hyponatremia → confusion, seizures, coma. Causes: small-cell lung cancer, CNS injury, certain drugs.
DI: not enough ADH (or kidneys ignore it)
The body loses water. Everything serum is concentrated; everything urine is dilute. Serum sodium HIGH (>145), serum osmolality high. Urine dilute and massive: specific gravity very low (around 1.005 or less), output that can reach many liters a day. Intense thirst, dehydration, hypotension, tachycardia. Central DI: the pituitary stops making ADH (head injury, pituitary surgery). Nephrogenic DI: kidneys stop responding (lithium is the classic drug cause).
Treatment: also mirror images
SIADH: fluid restriction (often 800-1000 mL/day) is the core intervention. Severe symptomatic hyponatremia: hypertonic 3% saline, slowly, because rapid correction damages the brain. Seizure precautions. Vaptans or demeclocycline in chronic cases. DI: replace water (oral or IV hypotonic fluids) and for central DI give desmopressin (DDAVP). Teach desmopressin patients to watch for the opposite problem: water retention and hyponatremia (headache, confusion). Nephrogenic DI: thiazides and low-salt diet, and stop the offending drug.
Nursing must-dos for both
Daily weights (the most sensitive fluid-balance measure: 1 kg is 1 L), strict intake and output, urine specific gravity, serum sodium trends, and neuro checks. Both appear after head injury and pituitary surgery, sometimes one after the other, so a neuro patient with sudden urine output changes gets sodium checked, not just charted.
Frequently asked
Post-craniotomy patient puts out 800 mL of pale urine in one hour. Which disorder?
DI. Massive dilute urine after pituitary or brain surgery. Check specific gravity and sodium and notify the provider.
Why is SIADH weight gain edema-free?
The retained water stays mostly inside cells and the vascular space rather than pooling in the interstitium, so the scale rises before the ankles swell.
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