SIADH vs Diabetes insipidus
How to tell SIADH and Diabetes insipidus apart when a board question makes them look alike.
SIADH: hyponatremia with inappropriately concentrated urine; diabetes insipidus: hypernatremia with dilute urine and polyuria.
Side by side
Hyponatremia with inappropriately concentrated urine and low serum osmolality.
Polyuria with dilute urine, high serum osmolality, and hypernatremia if water access cannot keep up.
Excess ADH effect retains free water.
Absent ADH effect or renal resistance causes free-water loss.
Low serum sodium plus high urine osmolality points to SIADH.
High or rising serum sodium plus low urine osmolality points to DI, especially when water access is limited.
The trap
Both are ADH problems but the direction of water movement is opposite. The classic test trap is the water deprivation test outcome: in central DI, urine osmolality stays low during dehydration but rises by more than 50 percent after exogenous desmopressin (DDAVP); in nephrogenic DI, urine osmolality stays low even after DDAVP because the collecting duct does not respond. In SIADH, restrict free water and treat the trigger; do NOT correct sodium faster than 8 to 12 mEq/L per 24 hours because rapid correction causes osmotic demyelination syndrome (central pontine myelinolysis).
Watch it explained
The repair move
For any sodium disorder, pair serum sodium with urine osmolality and urine sodium before naming the disorder. Hyponatremia with concentrated urine (urine osm above 100) plus elevated urine sodium (above 40) plus euvolemia equals SIADH (fluid restriction, treat underlying cause, demeclocycline or tolvaptan in refractory cases). Hypernatremia with dilute urine (urine osm under 300) equals diabetes insipidus; do water deprivation test followed by DDAVP to split central (urine osm rises after DDAVP) from nephrogenic (urine osm stays low after DDAVP) DI.
Try a free question
CoreA patient with small cell lung cancer has hyponatremia, low serum osmolality, and inappropriately concentrated urine. Which diagnosis is most likely?
Show answer and explanation
Answer: A. SIADH. Low serum sodium with concentrated urine from excess ADH effect points to SIADH.
Why the other choices are tempting
- B. DI causes free-water loss with dilute urine and often hypernatremia.
- C. Primary polydipsia causes excess water intake with dilute urine.
- D. Prerenal azotemia is an AKI physiology pattern.
- E. Adrenal crisis may cause hyponatremia but does not classically cause concentrated urine from ectopic ADH.
Flashcards
Tap a card to flip it.
What sodium pattern separates SIADH from DI?
SIADH causes dilutional hyponatremia; DI causes free-water loss with hypernatremia when intake cannot keep up.
What urine osmolality pattern supports SIADH?
Urine remains inappropriately concentrated despite low serum osmolality.
Quick answers
What is the fastest way to tell SIADH from Diabetes insipidus?
SIADH: hyponatremia with inappropriately concentrated urine; diabetes insipidus: hypernatremia with dilute urine and polyuria.
What is the key clue for SIADH?
Hyponatremia with inappropriately concentrated urine and low serum osmolality.
What is the key clue for Diabetes insipidus?
Polyuria with dilute urine, high serum osmolality, and hypernatremia if water access cannot keep up.
What is the common trap with SIADH vs Diabetes insipidus?
Both are ADH problems but the direction of water movement is opposite. The classic test trap is the water deprivation test outcome: in central DI, urine osmolality stays low during dehydration but rises by more than 50 percent after exogenous desmopressin (DDAVP); in nephrogenic DI, urine osmolality stays low even after DDAVP because the collecting duct does not respond. In SIADH, restrict free water and treat the trigger; do NOT correct sodium faster than 8 to 12 mEq/L per 24 hours because rapid correction causes osmotic demyelination syndrome (central pontine myelinolysis).