Compare conditions

Put any two — or three — conditions side by side, adult or pediatric, to spot the look-alike differences fast, row by row.

Diabetes Insipidus & SIADH

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In one line

·DI happens when your body makes too little ADH or your kidneys ignore it, so water floods out in your pee.

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Normal physiology

·Your brain watches your blood's saltiness (sodium level) every second. When blood gets too salty or volume drops, a tiny gland deep in your brain (the pituitary) releases ADH into your blood. ADH travels to your kidneys and tells the collecting ducts (the final tubes before urine leaves) to open water doors and pull water back into your blood, so you pee less and keep water. When blood is too watery, your brain releases less ADH, kidneys let water go, and you pee more. This loop keeps your blood sodium and water balanced every minute of every day.

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What goes wrong

·The ADH system breaks in two main ways. In central DI, your brain stops making enough ADH — usually because of brain surgery, a head injury, a tumor pressing on the pituitary, or (rarely) a genetic problem. In nephrogenic DI, your brain makes plenty of ADH, but your kidneys ignore it — often caused by long-term lithium (a mood medicine), low potassium, high calcium, kidney diseases, or (rarely) a genetic defect in the water channels. In SIADH, the opposite happens: your body releases too much ADH when it should not — triggered by brain injuries (bleeds, infections, tumors), lung problems (pneumonia, small-cell lung cancer), some cancers that make their own ADH, or medicines like SSRIs, carbamazepine, and certain chemo drugs.

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Hallmark signs

·Making huge amounts of very pale, watery pee (polyuria) — often 3 to 20 liters a day
·Constant, strong thirst (polydipsia) — needing to drink water all day and night
·Waking up multiple times at night to pee and drink (nocturia)
·Dry mouth and skin; feeling dehydrated
·Confusion, irritability, or sleepiness (in SIADH)
·Nausea and headache (in SIADH)
·Normal blood pressure and no visible swelling of the legs or belly (in SIADH)
·Seizures or coma (severe SIADH)

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Red flags · escalate now

·Sodium below 120 mEq/L — brain swelling risk is high; person may seize or become unresponsive
·Severe confusion, seizure, or loss of consciousness — emergency; needs immediate hospital care and careful, slow correction of sodium
·Unable to drink or keep up with urine output in diabetes insipidus — leads to dangerous dehydration, low blood pressure, and shock

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Workup

·Serum sodium
·Plasma osmolality
·Urine osmolality
·24-hour urine volume
·Water deprivation test (also called desmopressin challenge)
·Serum ADH (vasopressin) level
·MRI of the brain (pituitary and hypothalamus)
·Medication review and serum lithium or calcium level

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Treatment

·For central DI: give desmopressin (DDAVP) as a nasal spray, oral tablet, or injection
·For nephrogenic DI: give a thiazide diuretic (hydrochlorothiazide 25–50 mg daily) and tell the person to eat a low-salt diet (< 2 g sodium per day)
·For nephrogenic DI: add amiloride (5–10 mg daily) if lithium is the cause
·For SIADH: restrict fluid intake to 800–1000 mL per day (less than urine output)
·For SIADH with severe symptoms (sodium < 120 mEq/L, seizures, confusion, coma): give 3% hypertonic saline IV at 1–2 mL/kg/hour
·For SIADH: give oral salt tablets (1–3 g sodium three times daily), urea powder (15–30 g twice daily), or a vasopressin receptor antagonist (tolvaptan 15–60 mg daily)
·Find and treat the root cause: pituitary surgery or tumor for central DI; stop lithium or fix high calcium for nephrogenic DI; treat cancer, pneumonia, or stop the offending drug (SSRIs, carbamazepine) for SIADH

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NCLEX trap

·High sodium in DI means the body lost too much water, not that it has too much salt. The kidneys are dumping water out faster than the body can replace it. The fix is to replace the lost water (free water) and give the missing hormone (DDAVP for central DI) or a thiazide diuretic with a low-salt diet (for nephrogenic DI). Restricting salt won't fix the water loss.
·DI = high sodium, watery pee, extreme thirst, not enough ADH or kidneys ignore ADH. SIADH = low sodium, concentrated pee, no thirst, too much ADH. Think of it this way: DI dries the body out (like a desert), SIADH floods the body with water (like a sponge holding too much water).
·Never drop sodium faster than 8-10 mEq/L in 24 hours. If sodium falls too fast, water rushes into brain cells and they swell (cerebral swelling (edema)), which can cause seizures, coma, or death. Slow and steady protects the brain.
·DDAVP only works in central DI (when the pituitary gland isn't making enough ADH). In nephrogenic DI, the kidneys ignore ADH, so DDAVP does nothing. Use a water deprivation test or desmopressin challenge to figure out which type you're treating: central gets DDAVP, nephrogenic gets thiazide diuretic, low-salt diet, and plenty of water.
·Excessive urination (Polyuria) (peeing a lot) plus high serum osmolality (thick blood) plus low urine osmolality (thin pee) = DI until proven otherwise. Always check blood and urine osmolality; that combination tells the real story. Blaming the patient for drinking too much misses the diagnosis and delays life-saving treatment.
·Central DI comes from damage to the pituitary gland or hypothalamus (surgery, trauma, tumor, infection, inflammation). Nephrogenic DI comes from kidney damage or resistance (drugs like lithium, chronic kidney disease, genetic defects). Always ask: is the break in the brain (central) or the kidneys (nephrogenic)?

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