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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- DI = polyuria with inappropriately dilute urine, often with polydipsia and hypernatremia if water access is limited
- Central DI = deficient ADH; nephrogenic DI = renal resistance to ADH
- Water deprivation test: urine remains dilute; desmopressin concentrates urine in central DI but not nephrogenic DI
- Common nephrogenic causes: lithium, hypercalcemia, hypokalemia, kidney disease
- Treat central DI with desmopressin; treat nephrogenic DI by removing cause, low-solute diet, thiazide, NSAID in selected patients
Overview
Diabetes insipidus is characterized by inability to concentrate urine due to absent vasopressin secretion or renal nonresponse. Patients produce large volumes of dilute urine and compensate with intense thirst. If thirst or water access is impaired, hypernatremia can develop rapidly. DI must be distinguished from primary polydipsia and osmotic diuresis from uncontrolled diabetes mellitus, hypercalcemia, high solute intake, or diuretics.
Epidemiology
Central DI may follow pituitary surgery, traumatic brain injury, tumors, infiltrative disease, autoimmune hypophysitis, ischemia, or idiopathic posterior pituitary dysfunction. Nephrogenic DI is most commonly acquired from lithium therapy, hypercalcemia, hypokalemia, chronic kidney disease, or medications such as demeclocycline and amphotericin B. Congenital nephrogenic DI is rare and often presents in infancy.
Clinical Features
Symptoms
Polyuria often >3 L/day in adults, nocturia, intense thirst
Preference for cold water
Dehydration, dizziness, weakness if unable to drink adequately
Confusion, seizures, or coma from severe hypernatremia
History of pituitary surgery, head trauma, lithium use, hypercalcemia, or hypokalemia
Signs
Very dilute urine with low specific gravity
Dry mucous membranes, tachycardia, hypotension if volume depleted
Hypernatremia when water access is limited
Pituitary mass signs: visual field loss, hypopituitarism
No glucosuria unless diabetes mellitus also present
Investigations
First-line
Serum sodium and plasma osmolalityHigh or high-normal sodium/osmolality supports DI over primary polydipsia
Urine osmolality and specific gravityInappropriately low urine osmolality despite serum hyperosmolality suggests DI
Glucose, calcium, potassium, creatinineExclude osmotic diuresis and reversible nephrogenic causes
Second-line
Water deprivation test with desmopressinUnder supervision: differentiates central DI, nephrogenic DI, and primary polydipsia
Copeptin testingIncreasingly used where available to distinguish AVP deficiency/resistance from primary polydipsia
24-hour urine volumeConfirms true polyuria
Specialist
Pituitary MRIFor central DI or suspected hypothalamic-pituitary lesion; posterior pituitary bright spot may be absent
Genetic testingIf congenital nephrogenic DI suspected
1
Acute hypernatremia or dehydration
- Restore intravascular volume with isotonic saline if hypotensive, then correct free water deficit carefully
- Avoid overly rapid sodium correction unless hypernatremia is clearly acute
- Monitor sodium and urine output frequently
2
Central DI
- Desmopressin intranasal, oral, or parenteral depending on severity and setting
- Educate about avoiding excessive water intake after desmopressin to prevent hyponatremia
- Treat underlying pituitary/hypothalamic cause
3
Nephrogenic DI
- Stop lithium or offending drug when possible; correct hypercalcemia or hypokalemia
- Low-sodium, low-solute diet to reduce urine volume
- Thiazide diuretic paradoxically reduces polyuria by inducing mild volume contraction and proximal sodium/water reabsorption
- NSAID such as indomethacin may reduce prostaglandin-mediated antagonism of ADH in selected patients
4
Primary polydipsia
- Avoid desmopressin unless specialist-directed because water intoxication and hyponatremia can occur
- Address psychiatric, behavioral, or medication drivers
Complications
- Hypernatremic dehydration: Especially if impaired thirst, infants, older adults, or hospitalized patients without water access
- Seizures and coma: Severe hypernatremia causes brain cell shrinkage
- Hyponatremia from overtreatment: Desmopressin plus excess water intake can cause dangerous water retention
- Lithium nephrotoxicity: Chronic lithium can cause persistent nephrogenic DI and CKD
- Pituitary disease: Central DI may coexist with anterior pituitary hormone deficiencies
USMLE Step 2 CK Exam Tips
- 1DI has dilute urine despite high serum osmolality; diabetes mellitus causes osmotic diuresis with glucosuria
- 2Central DI responds to desmopressin with increased urine osmolality; nephrogenic DI does not
- 3Lithium is the classic cause of nephrogenic DI
- 4Hypercalcemia and hypokalemia can both cause nephrogenic DI
- 5After pituitary surgery, abrupt polyuria with hypernatremia = central DI
- 6Thiazides treat nephrogenic DI paradoxically by reducing distal delivery of filtrate
- 7Primary polydipsia can look similar but serum sodium tends to be low-normal rather than high
practicetest your knowledge on diabetes insipidus (central vs nephrogenic)Apply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — endocrine and beyond.
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