What is the difference between Diabetes Insipidus (DI) and Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)?

Diabetes Insipidus vs SIADH: Key Differences

Diabetes insipidus (DI) and SIADH represent opposite ends of the spectrum of water balance disorders—DI causes excessive free water loss with dilute urine and hypernatremia, while SIADH causes excessive water retention with concentrated urine and hyponatremia. 1

Fundamental Pathophysiology

Diabetes Insipidus

• DI results from either inadequate ADH production (central DI) or renal insensitivity to ADH (nephrogenic DI), leading to inability of the kidneys to reabsorb water 2, 3
• Patients excrete abnormally large volumes of dilute urine (>2.5 L per 24 hours) with urine osmolality <200 mOsm/kg H₂O 4, 2
• The hallmark triad is polyuria, polydipsia, and inappropriately dilute urine combined with high-normal or elevated serum sodium 5, 4
• Free water loss leads to hypernatremia and volume depletion if fluid intake is inadequate 1

SIADH

• SIADH results from excessive or inappropriate ADH secretion, causing the kidneys to retain too much water 1
• This leads to hypotonic hyponatremia that develops insidiously 1
• Urine remains inappropriately concentrated (high urine osmolality) despite low serum osmolality
• Patients develop dilutional hyponatremia without volume depletion 1

Clinical Presentation Differences

Diabetes Insipidus Presentation

• Polyuria (excessive urine output) 2, 6
• Polydipsia (excessive thirst) 2, 6
• Hypernatremia (elevated serum sodium) 5, 1
• Dehydration if fluid intake cannot match losses 1
• In children: failure to thrive and hypernatremic dehydration 4

SIADH Presentation

• Hyponatremia (low serum sodium) 1
• Concentrated urine despite low serum osmolality
• Euvolemic or mildly hypervolemic state
• Neurological symptoms related to hyponatremia (confusion, seizures in severe cases)

Diagnostic Approach

For Diabetes Insipidus

• Measure serum sodium, serum osmolality, and urine osmolality simultaneously as initial work-up 5, 4
• The combination of urine osmolality <200 mOsm/kg H₂O with high-normal or elevated serum sodium confirms DI 4
• Plasma copeptin levels distinguish subtypes: >21.4 pmol/l suggests nephrogenic DI, while <21.4 pmol/l indicates central DI or primary polydipsia 7, 5
• Water deprivation test followed by desmopressin administration remains the gold standard 6

For SIADH

• Low serum sodium with low serum osmolality
• Inappropriately concentrated urine (high urine osmolality relative to serum osmolality)
• Euvolemic clinical status
• Exclusion of other causes of hyponatremia (hypothyroidism, adrenal insufficiency, diuretic use)

Management Differences

Diabetes Insipidus Management

• Free access to fluid is essential in all DI patients to prevent dehydration, hypernatremia, growth failure, and constipation 5, 4
• For central DI: desmopressin is the treatment of choice 4
• For nephrogenic DI: thiazide diuretics combined with prostaglandin synthesis inhibitors (NSAIDs) for symptomatic patients 5, 4
• Low-salt diet (≤6 g/day) and protein restriction (<1 g/kg/day) to reduce renal osmotic load in nephrogenic DI 5, 4
• Patients capable of self-regulation should determine fluid intake based on thirst rather than prescribed amounts 5, 4

SIADH Management

• Fluid restriction to 1L/day is the cornerstone of SIADH treatment 8
• Daily weight monitoring and serum sodium measurement every 4 hours 8
• In severe symptomatic hyponatremia: hypertonic saline may be required
• Treatment of underlying cause when identified

Critical Pitfalls to Avoid

In Diabetes Insipidus

• Never restrict fluids in DI patients—this causes life-threatening hypernatremia and dehydration 5, 4
• When patients require fasting (>4 hours), provide intravenous 5% dextrose in water at maintenance rate with close monitoring 5
• Calculate initial IV fluid rate to avoid decreasing serum sodium >8 mmol/l/day in hypertonic dehydration 7
• Medical staff often fail to appreciate the specific fluid requirements of hospitalized DI patients who cannot maintain adequate oral intake 7

In SIADH

• Avoid overly rapid correction of hyponatremia (risk of osmotic demyelination syndrome)
• Do not give hypotonic fluids, which worsen hyponatremia
• Recognize that fluid restriction is therapeutic in SIADH but lethal in DI

Emergency Considerations

For Diabetes Insipidus Patients

• Each DI patient should have an emergency plan including a letter explaining their diagnosis with IV fluid management advice and contact information 7, 5
• Medical alert bracelet or card is recommended 7
• Use water with dextrose or glucose (5% dextrose) for IV rehydration, not normal saline 7, 5
• Close observation of neurological status, fluid balance, body weight, and serum electrolytes is mandatory 7, 5
• Patients with hypernatremic dehydration should be treated in specialized centers or under expert consultation 7