SIADH vs Diabetes Insipidus & Hyponatremia: Step 1 review notes and self-test
Notes for the SIADH vs Diabetes Insipidus & Hyponatremia rapid review: the clue that separates each of its 4 confusion pairs, with timestamps and a self-test.
October 21, 2026
This video covers SIADH, diabetes insipidus, hyponatremia volume patterns, and major diuretic differences. Watch the video, then use these notes to test yourself.
SIADH vs Diabetes insipidus
SIADH causes hyponatremia with inappropriately concentrated urine and low serum osmolality, while diabetes insipidus causes polyuria with dilute urine, high serum osmolality, and possible hypernatremia. Both are ADH problems, but water movement goes in opposite directions. In central diabetes insipidus, urine osmolality rises by more than 50 percent after desmopressin, while in nephrogenic diabetes insipidus it stays low. In SIADH, do not correct sodium faster than 8 to 12 mEq/L per 24 hours because rapid correction causes osmotic demyelination syndrome.
SIADH vs Diabetes insipidus SIADH: hyponatremia with inappropriately concentrated urine; diabetes insipidus: hypernatremia with dilute urine and polyuria. Open the comparison and video →Central diabetes insipidus vs Nephrogenic diabetes insipidus
Central diabetes insipidus has low ADH production with intact kidney responsiveness, while nephrogenic diabetes insipidus has renal resistance to ADH. Both can cause polyuria above 3 L per day, urine osmolality below 300, and hypernatremia when water access is limited. After desmopressin, urine osmolality rises by more than 50 percent in central diabetes insipidus but stays low in nephrogenic diabetes insipidus.
Central diabetes insipidus vs Nephrogenic diabetes insipidus Central DI responds to desmopressin with urine concentration; nephrogenic DI stays dilute because the kidney cannot respond. Open the comparison and video →Hypovolemic hyponatremia vs Euvolemic / hypervolemic hyponatremia
After you prove hypotonicity, use the volume exam. Dry findings such as orthostasis, dry mucosa, and tachycardia point to hypovolemic hyponatremia, a normal exam points to SIADH-type euvolemic hyponatremia, and edema or ascites points to hypervolemic hyponatremia. The trap is that low sodium alone does not tell you the volume status, so use urine concentration and the bedside volume exam.
Hypovolemic hyponatremia vs Euvolemic / hypervolemic hyponatremia First prove hypotonicity, then use volume exam: dry is hypovolemic, normal is SIADH-type euvolemic, swollen is hypervolemic. Open the comparison and video →Loop diuretics vs Thiazide and potassium-sparing diuretics
Loop diuretics block NKCC2, cause strong diuresis and calcium loss, and carry an ototoxicity risk. Thiazides retain calcium, while potassium-sparing drugs carry a hyperkalemia risk. Loop and thiazide drugs can both cause hypokalemic metabolic alkalosis, so use calcium handling and ototoxicity to separate them.
Loop diuretics vs Thiazide and potassium-sparing diuretics Loop means NKCC2, strong diuresis, calcium wasting, ototoxicity; thiazide means calcium retention; potassium-sparing means hyperkalemia. Open the comparison and video →Quick self-test
Name the diagnosis from the clue, then tap to check.
Low ADH production from hypothalamic or posterior pituitary dysfunction with intact renal responsiveness.
Central diabetes insipidus · Central diabetes insipidus vs Nephrogenic diabetes insipidus
Hyponatremia with inappropriately concentrated urine and low serum osmolality.
SIADH · SIADH vs Diabetes insipidus
Polyuria with dilute urine, high serum osmolality, and hypernatremia if water access cannot keep up.
Diabetes insipidus · SIADH vs Diabetes insipidus
Furosemide/bumetanide pattern with powerful diuresis from thick ascending limb NKCC2 blockade, calcium loss, hypokalemia, and ototoxicity risk.
Loop diuretics · Loop diuretics vs Thiazide and potassium-sparing diuretics
Renal resistance to ADH, classically from lithium, hypercalcemia, hypokalemia, or inherited collecting-duct defects.
Nephrogenic diabetes insipidus · Central diabetes insipidus vs Nephrogenic diabetes insipidus
Keep going
Every pair in this review has a free page in the library with a side-by-side comparison and a practice question. Work through the ones you missed in the self-test first.