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

SIADH · Syndrome of Inappropriate ADH

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In one line

·The body releases too much ADH hormone even when it should not, so the kidneys hold extra water and blood salt drops dangerously low.

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Normal physiology

·Normally, special nerve cells in the hypothalamus (a control center deep in the brain) measure how concentrated your blood is. When blood gets too salty or thick, those cells tell the pituitary gland (a pea-sized hormone factory hanging under the brain) to release ADH into the bloodstream. ADH travels to the kidneys and orders the collecting ducts (the final tubes where urine is finished) to pull water back into the blood, making urine more concentrated and blood more dilute. When blood salt drops or gets too watery, the hypothalamus shuts off ADH so the kidneys dump extra water as dilute, pale urine and salt climbs back to normal. This feedback loop keeps sodium between 135 and 145 milliequivalents per liter — the narrow range cells need to work correctly.

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What goes wrong

·In SIADH, something hijacks ADH release so it keeps flowing even when blood is already too watery and sodium is low. The kidneys obey ADH and hold water they should dump, diluting blood further. The break is not in the kidney or the sodium itself — it is in the control switch that should turn ADH off but does not.

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Hallmark signs

·Low sodium in the blood (hyponatremia)
·Nausea and vomiting
·Headache
·Confusion or trouble thinking clearly
·Muscle cramps or weakness
·Seizures
·Lethargy or extreme sleepiness
·Concentrated urine (dark or strong-smelling) despite drinking enough water

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Red flags · escalate now

·Seizure (sudden shaking or loss of awareness)
·Severe confusion or not responding when you speak to the person
·Sodium level below 120 mEq/L or dropping very fast (more than 10 points in 24 hours)
·Trouble breathing or very slow breathing
·Coma (cannot be woken up)

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Workup

·Serum sodium
·Serum osmolality
·Urine osmolality
·Urine sodium
·TSH (thyroid-stimulating hormone)
·Morning cortisol or ACTH stimulation test
·BUN (blood urea nitrogen) and creatinine
·Chest X-ray or CT chest

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Treatment

·Fluid restriction to 800–1000 mL per day (all liquids — water, juice, IV fluids, soup)
·Treat the underlying cause — remove tumor, stop the triggering medicine (SSRI, carbamazepine, NSAID, cyclophosphamide), or treat lung infection
·3% hypertonic saline IV, 100 mL bolus over 10 minutes, for severe symptoms (seizure, coma, or sodium below 120 mEq/L)
·Oral sodium chloride tablets (1–2 grams three times a day) plus loop diuretic (furosemide 20–40 mg once or twice daily)
·Tolvaptan (a vasopressin V2-receptor antagonist, or 'vaptan') 15 mg once daily, only in hospital
·Urea 15–60 grams per day (powder mixed in juice or water), mainly used in Europe and Canada

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NCLEX trap

·In SIADH, normal saline can actually make the sodium drop even more because the kidneys will keep holding onto water and dump the salt into the urine. Fluid restriction (limit fluids to 800–1000 mL per day) is the first treatment. Only use 3% hypertonic saline (very salty IV fluid) if the patient is having seizures or the sodium is dangerously low (below 120 mEq/L).
·Salt tablets alone will not fix SIADH because the kidneys will just filter that extra salt right into the urine. You must restrict fluids first. If fluid restriction and salt tablets together do not work, you can add tolvaptan (a drug that blocks ADH from telling the kidneys to hold water).
·Both hypothyroidism and adrenal insufficiency can cause low sodium that looks like SIADH, but they need completely different treatments (thyroid hormone or steroids). Always check TSH (thyroid test) and a morning cortisol level before you confirm SIADH.
·Correcting sodium too quickly can cause osmotic demyelination syndrome (the protective coating around brain nerves gets damaged). Raise sodium only 4–6 mEq/L in the first 6 hours, and never more than 8 mEq/L in the first 24 hours total.
·Fixing the low sodium without fixing why ADH is too high means SIADH will come right back. Always search for and treat the root cause: stop the culprit drug, treat the infection, or work up a possible cancer.
·Tolvaptan can damage the liver. The FDA says to use it only in the hospital where you can check liver enzyme tests (AST, ALT) frequently. Use it only if fluid restriction and salt tablets have failed.

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