Diabetes Insipidus (DI)

Definition

Pathophysiology & Classification

1. Central (Neurogenic) DI

• The hypothalamus-posterior pituitary fails to secrete vasopressin (ADH) in response to elevated osmolality
• As few as 10-15% of normal vasopressinergic hypothalamic neurons is sufficient to prevent symptoms - loss of even a small fraction beyond this triggers rapid symptomatic polyuria
• Without ADH, the collecting duct cannot insert aquaporin water channels (AQP2) into luminal membranes, so dilute filtrate is excreted unchanged
• Plasma vasopressin is unmeasurable or inappropriately low for the plasma osmolality
• Causes: tumors, head injury, hemorrhage, neurosurgery (especially transsphenoidal pituitary resection), neurosarcoidosis, histiocytosis, tuberculous meningitis, multiple sclerosis, or idiopathic
• Autosomal dominant hereditary forms exist (mutations in the signal peptide or neurophysin portion of the pre-prohormone)

2. Nephrogenic DI

• The kidney does not respond to normal or elevated ADH levels
• Congenital:

  • 90% - mutations in the AVPR2 gene (V2 vasopressin receptor) - X-linked, prevalence ~4-8 per million male births

  • <10% - mutations in the AQP2 gene (aquaporin 2 water channel) - autosomal dominant or recessive
• Acquired (more common than congenital):
  • Lithium (most common drug cause - interferes with AQP2 insertion)
  • Hypercalcemia
  • Hypokalemia
  • Ureteric obstruction
  • CKD
  • Demeclocycline
• Goldman-Cecil Medicine, p. 1204; Tietz Textbook, p. 1895

3. Osmoreceptor Dysfunction

4. Primary Polydipsia (not true DI, but important differential)

• Psychogenic polydipsia - excess intake due to psychiatric illness (schizophrenia, mania, OCD); prevalence up to 40% in psychiatric hospitals
• Dipsogenic DI - resetting of thirst threshold below the ADH secretion threshold; idiopathic or from hypothalamic lesions
• Sustained high fluid intake washes out the medullary concentrating gradient, so patients may fail to concentrate urine even with vasopressin administration

5. Gestational DI

• Increased placental vasopressinase degrades circulating ADH during pregnancy
• Responds to desmopressin (which is resistant to vasopressinase)

Clinical Features

• If thirst is also impaired (damage to thirst center co-localised with hypothalamus), dangerous hyperosmolarity and irreversible brain damage can result
• In children with congenital disease from birth: massive dilatation of the renal pelvis, ureters, and bladder can develop - Goldman-Cecil Medicine, p. 1205

Diagnosis

• Uosm <300 mOsm/kg = hypotonic (water diuresis) → consider DI or primary polydipsia
• Uosm >300 mOsm/kg = solute diuresis → investigate for hyperglycemia, azotemia, etc.

• ≥21.4 pmol/L → confirms partial or complete nephrogenic DI
• <2.6 pmol/L → confirms complete central DI
• Intermediate → proceed to water deprivation test or hypertonic saline infusion test

Diagnostic flowchart for polyuria - Goldman-Cecil Medicine

1. Withhold all fluid; collect hourly urine volume and osmolality; monitor weight and BP
2. Stop when Uosm plateaus (<30 mOsm/kg rise over 3 consecutive hours), weight drops 3-5%, or BP falls >20 mmHg systolic
3. Measure plasma vasopressin, then administer 2 µg desmopressin IV/SC
4. Measure Uosm at 30, 60, 120 min after injection

• Henry's Clinical Diagnosis and Management, p. 735

Treatment

Central DI

• Desmopressin (DDAVP) - synthetic vasopressin V2 receptor agonist, first-line treatment
  • Routes: intranasal (10-40 µg/day), oral (0.1-1.2 mg/day), or parenteral
  • Eliminates nocturia; reduces urine volume dramatically
  • Risk: hyponatremia if fluid intake is not reduced appropriately
• Carbamazepine, chlorpropamide, clofibrate - augment residual ADH secretion in partial DI

Nephrogenic DI

• Remove the causative agent if possible (e.g., stop lithium, correct hypercalcemia or hypokalemia)
• Low-sodium, low-protein diet - reduces solute load delivered to collecting duct
• Thiazide diuretics (e.g., hydrochlorothiazide) - cause mild volume depletion → increase proximal tubular reabsorption → less fluid reaches the collecting duct → reduced urine output (paradoxical antidiuretic effect)
• Amiloride - blocks lithium entry into principal cells; useful for lithium-induced NDI
• NSAIDs (indomethacin) - reduce prostaglandin synthesis → enhance the limited tubular response to ADH
• Desmopressin generally has little/no effect in complete nephrogenic DI

Primary Polydipsia

• Fluid restriction
• Treat underlying psychiatric illness

Key Distinguishing Points: DI vs. Diabetes Mellitus

Recent Evidence (2024-2026)

• Copeptin for diagnosing DI in children (PMID 39409072) confirms copeptin's diagnostic utility in pediatric polyuria-polydipsia syndrome, supporting the shift away from the classical water deprivation test
• Copeptin post-pituitary surgery (PMID 39393633) supports early postoperative copeptin measurement for predicting DI after pituitary surgery - important for surgical practice
• Gestational DI (PMID 40440645) - a 2025 systematic review of case reports characterising this rare pregnancy-related form

🩺 Patient Education Sheet

Diabetes Insipidus (DI) - Understanding Your Condition

What Is Diabetes Insipidus?

Why Does It Happen?

Type 1 - Central DI

• A head injury or brain surgery
• A tumour near the pituitary gland or hypothalamus
• An infection or inflammation in the brain
• Sometimes no clear cause is found (idiopathic)

Type 2 - Nephrogenic DI

• Certain medications - especially lithium (used for mood disorders)
• High calcium levels in the blood
• Low potassium levels
• An inherited (genetic) condition
• Chronic kidney disease

Symptoms - What to Watch For

Common Symptoms

• Passing very large amounts of urine - often 4-20 litres per day
• Extreme thirst - especially a craving for cold or iced drinks
• Waking at night to urinate (nocturia) - this is very common and disrupts sleep
• Pale or colourless urine

In Children, Also Watch For

• Unexplained crying or irritability
• Slow weight gain or poor growth
• High fever without an obvious cause
• Preferring cold drinks to warm ones

Warning Signs - Seek Medical Help Immediately If You Notice

• Confusion or difficulty thinking clearly
• Severe headache
• Nausea or vomiting with low urine output
• Signs of dehydration: dry mouth, sunken eyes, dizziness, rapid heartbeat
• Inability to keep up with fluid losses (e.g., during illness or heat)

Important: If your thirst mechanism is impaired (your doctor will tell you if this applies), you are at higher risk of dangerous dehydration. You must drink on a strict schedule, not just when you feel thirsty.

How Is It Diagnosed?

• Urine tests - to measure how concentrated your urine is
• Blood tests - to measure sodium and osmolality (the concentration of your blood)
• Copeptin blood test - a newer, reliable test that measures a substance released alongside ADH
• Water deprivation test - done in hospital under supervision; fluid is withheld for a period to see how your kidneys respond
• MRI of the brain - to look at the pituitary gland and hypothalamus

Treatment - Managing Your DI

For Central DI (brain does not make enough ADH)

• Available as a nasal spray, tablet, or injection
• Usually taken once or twice a day
• You should notice a big reduction in urine output within hours

• Take it at the same time each day
• Do not drink extra fluid beyond what your thirst calls for - too much fluid with desmopressin can cause dangerously low sodium (hyponatremia)
• Tell your doctor if you develop headache, nausea, or swelling - these may be signs of too much fluid retention
• If you are unwell and cannot eat or drink, contact your medical team - your dose may need adjusting

For Nephrogenic DI (kidneys do not respond to ADH)

• Identify and remove the cause - if a medication such as lithium is responsible, your doctor will discuss whether it can be stopped or reduced. Do not stop any medication without advice.
• Low-salt, low-protein diet - reduces the workload on your kidneys; a dietitian can help with this
• Thiazide diuretics (a type of water tablet, e.g., hydrochlorothiazide) - paradoxically reduce urine volume by causing mild volume depletion
• Amiloride - sometimes added, especially for lithium-related DI
• Anti-inflammatory drugs (NSAIDs) such as indomethacin - used in some cases to help the kidneys respond better

Living With DI - Day-to-Day Tips

Hydration

• Carry a water bottle at all times
• In hot weather or during exercise, increase your fluid intake to match what you lose - discuss specific targets with your doctor
• Avoid alcohol and caffeine where possible - both increase urine production

Diet

• A low-salt diet helps reduce urine output in both types
• Avoid very high-protein meals - protein breakdown increases the load on the kidneys
• If on desmopressin, do not drink excessive fluid "just in case" - this is dangerous

Sleep

• Nocturia (waking to urinate at night) is common. Taking desmopressin in the evening can help significantly.
• Keep a water bottle by the bed in case you wake thirsty

Travel

• Always carry your medication (desmopressin) in your hand luggage when flying
• Carry a letter from your doctor explaining your condition and medications - especially for travel with injectable forms
• Inform travel companions about your condition and what to do if you become unwell

Work and Daily Life

• Access to a toilet at all times is important - discuss reasonable adjustments with your employer if needed
• You can live a normal, active life with well-managed DI

Medical Alert

• Consider wearing a medical alert bracelet or carrying a card stating you have diabetes insipidus. Emergency staff need to know - DI can be confused with other conditions.

Monitoring - What Your Doctor Will Check

Frequently Asked Questions

When to Call Your Doctor

• A sudden increase in urine output or thirst
• Signs of dehydration (dizziness, dry mouth, confusion)
• Swelling, sudden weight gain, or headache (may indicate too much desmopressin)
• You are unable to take your medication due to illness or vomiting
• Any new symptoms you are unsure about

Your Healthcare Team

ADH - Antidiuretic Hormone (Vasopressin)

What It Is

Where It Is Made and Released

• Supraoptic nucleus (SON)
• Paraventricular nucleus (PVN)

1. Vasopressin (9 amino acids) - the active hormone
2. Neurophysin II (95 amino acids) - a carrier protein that stabilises ADH
3. Copeptin (39 amino acids) - a glycopeptide released in equimolar amounts with ADH; used as a surrogate marker in blood tests because it is more stable than ADH itself

• Goldman-Cecil Medicine, p. 2417

What Triggers Its Release

How It Works - The Mechanism of Action

V2 Receptors (main antidiuretic action) - on kidney collecting duct cells

1. ADH binds to V2 receptors on principal cells of the collecting duct
2. This activates adenylyl cyclase via Gs protein → increases intracellular cAMP
3. cAMP activates protein kinase A, which triggers the synthesis and insertion of aquaporin-2 (AQP2) water channels into the luminal (apical) membrane of the cell
4. Water moves from the dilute tubular fluid into the hypertonic medullary interstitium, down the osmotic gradient
5. Result: concentrated, small-volume urine

V1 Receptors - on vascular smooth muscle

• At high concentrations, ADH binds V1 receptors → activates phospholipase C via Gq protein → vasoconstriction
• This is why it is also called "vasopressin" - relevant in haemorrhagic shock management

Extrarenal V2-like Receptors

• Regulate release of coagulation factor VIII and von Willebrand factor - used clinically (desmopressin given to treat mild haemophilia A and von Willebrand disease)
• Comprehensive Clinical Nephrology 7th Ed., p. 127; Katzung, p. 1063

Normal Function Summary

High plasma osmolality
         ↓
Hypothalamus osmoreceptors activated
         ↓
ADH released from posterior pituitary
         ↓
ADH binds V2 receptors on collecting duct
         ↓
AQP2 water channels inserted into luminal membrane
         ↓
Water reabsorbed back into blood
         ↓
Concentrated urine produced
Plasma osmolality returns to normal

What Happens When ADH Is Absent or Ineffective

Key Facts at a Glance