SIADH vs Diabetes Insipidus: Key Differences for the NCLEX

By an RN · High-yield endocrine comparison

The 10-second versionSIADH = too much ADH → body holds water → fluid overload + LOW sodium + concentrated urine. Diabetes insipidus (DI) = too little ADH (or kidneys ignore it) → body dumps water → fluid deficit + HIGH sodium + huge amounts of dilute urine. They are mirror images.

What ADH does

Antidiuretic hormone (ADH, vasopressin) tells the kidneys to reabsorb water. More ADH = less urine, more concentrated. Less ADH = more urine, very dilute. Every SIADH-vs-DI question is really an ADH question.

Side-by-side comparison

	SIADH	Diabetes Insipidus
ADH level	Too much	Too little (central) or kidneys don't respond (nephrogenic)
Fluid status	Overload — weight gain, crackles, bounding pulses	Deficit — weight loss, poor skin turgor, hypotension, tachycardia
Sodium	Low (dilutional hyponatremia)	High (hemoconcentration)
Urine	Small amounts, concentrated (high osmolality)	Large amounts, dilute (low osmolality, low specific gravity)
Thirst	Not notably thirsty	Extreme thirst (polydipsia)
Common causes	CNS disorders, small-cell lung cancer, certain drugs	Head trauma, pituitary surgery, lithium (nephrogenic)
Treatment	Fluid restriction; severe hyponatremia → hypertonic saline	Replace fluids; desmopressin (DDAVP) for central DI

What the NCLEX actually asks

Priority intervention for SIADH: fluid restriction. (The test loves this — the overloaded patient does not get more IV fluids.)
Priority intervention for DI: replace fluids and give DDAVP (central DI). Monitor for dehydration and hypernatremia.
Assessments for both: strict I&O, daily weights, neuro status, serum sodium and osmolality. Seizure risk exists on both sides — from hyponatremia in SIADH, from hypernatremia and dehydration in DI.
Trap answer: giving hypotonic fluids to the SIADH patient "for the low sodium" — wrong. Restrict fluids first.

Memory trick

SIADH = Soaked Inside (waterlogged). DI = Dry Inside (dumping water). Sodium follows the water in reverse: soaked → diluted → low sodium; dry → concentrated → high sodium.

Want this as a printable study card?
Get the free 8-page NCLEX sample pack — 10 high-yield cards just like this topic.
Get the free sample pack