Diabetes insipidus vs Primary polydipsia
How to tell Diabetes insipidus and Primary polydipsia apart when a board question makes them look alike.
DI loses water despite need to conserve it; primary polydipsia drinks excess water and can concentrate when intake is withheld.
Side by side
Polyuria and polydipsia from ADH deficiency or renal ADH resistance, often with hypernatremia and dilute urine when water access is limited.
Excess water intake, often behavioral or psychiatric, causing polyuria with low or low-normal serum sodium and ability to concentrate urine after water deprivation.
Central DI lacks ADH release; nephrogenic DI has kidney resistance to ADH, so collecting ducts fail to reabsorb free water.
High water intake suppresses ADH and washes out medullary concentration gradient without primary ADH pathway failure.
High serum osmolality or hypernatremia with inappropriately dilute urine supports DI; desmopressin response separates central from nephrogenic.
Low serum osmolality, low sodium tendency, dilute urine, and urine concentration after water deprivation support primary polydipsia.
The trap
Both cause polyuria with dilute urine, so serum sodium/osmolality and water deprivation response decide.
The repair move
For dilute polyuria, decide whether the body is losing water pathologically or drinking water excessively.
Try a free question
CoreA patient has intense thirst, large-volume dilute urine, serum sodium of 151 mEq/L, high serum osmolality, and inappropriately low urine osmolality. Which disorder category best fits?
Show answer and explanation
Answer: A. Diabetes insipidus. Hypernatremia with high serum osmolality and dilute urine points to diabetes insipidus.
Why the other choices are tempting
- B. Primary polydipsia usually lowers serum sodium or serum osmolality from excess intake.
- C. SIADH causes hyponatremia with inappropriately concentrated urine.
- D. Type 2 diabetes causes osmotic diuresis from hyperglycemia, not isolated ADH-pathway water loss.
- E. Conn syndrome causes hypertension and hypokalemic metabolic alkalosis.
Flashcards
Tap a card to flip it.
What sodium pattern favors diabetes insipidus?
Hypernatremia or high serum osmolality favors diabetes insipidus when urine stays dilute.
What sodium pattern favors primary polydipsia?
Low or low-normal sodium with excess water intake favors primary polydipsia.
Quick answers
What is the fastest way to tell Diabetes insipidus from Primary polydipsia?
DI loses water despite need to conserve it; primary polydipsia drinks excess water and can concentrate when intake is withheld.
What is the key clue for Diabetes insipidus?
Polyuria and polydipsia from ADH deficiency or renal ADH resistance, often with hypernatremia and dilute urine when water access is limited.
What is the key clue for Primary polydipsia?
Excess water intake, often behavioral or psychiatric, causing polyuria with low or low-normal serum sodium and ability to concentrate urine after water deprivation.
What is the common trap with Diabetes insipidus vs Primary polydipsia?
Both cause polyuria with dilute urine, so serum sodium/osmolality and water deprivation response decide.