Nursing care
Why too much ADH dilutes sodium without obvious oedema
Written and reviewed by Elena Vasquez, BSN, RN, CMSRN · 4 min read · Updated October 2026
Short answer
SIADH causes hyponatraemia because too much antidiuretic hormone makes the kidneys hold on to free water. The retained water dilutes the sodium already in the blood, so serum sodium falls even though total body sodium is near normal. Urine stays concentrated and scanty. The main danger is water shifting into brain cells, so neuro checks, fluid restriction and careful correction matter.
Excess ADH keeps free water in the body
ADH, also called vasopressin, is made in the hypothalamus and released by the pituitary. It tells the kidney collecting ducts to reabsorb water. MedlinePlus explains that in SIADH the body makes too much ADH, so it retains too much water. Causes include some medicines, lung disease, brain injury or infection, surgery and certain cancers.
Normally a falling sodium switches ADH off so the kidneys can excrete dilute urine. In SIADH that brake fails. MSD defines the syndrome as urine that is less than maximally dilute despite a low plasma osmolality, without volume depletion or overload. The kidney keeps saving water the body does not need.
Dilution, not sodium loss, lowers the level
Serum sodium is a concentration, not a total amount. When extra water is added to the same amount of sodium, the concentration falls. MSD describes hyponatraemia as an excess of water relative to solute and places SIADH among the causes of euvolaemic hyponatraemia, in which total body water rises while total body sodium stays close to normal.
Oedema is usually absent because the mild volume expansion makes the kidneys excrete more sodium, and about two thirds of the retained water moves into cells, so blood pressure and skin turgor may look unremarkable. This is a common exam trap: a low sodium without swelling is not the same as dehydration. Giving more free water would deepen the dilution.
Why the urine is concentrated and the brain swells
Because ADH keeps pulling water back, urine output tends to fall and the urine is concentrated, with a high osmolality relative to the blood and often a raised specific gravity. MedlinePlus states that urine in SIADH is very concentrated. Intake and output that show intake well above output support the picture.
As plasma becomes more dilute, water moves by osmosis into cells, including brain cells. MSD explains that the neurologic features come from this shift: headache, confusion, stupor and, when severe or rapid, seizures and coma. Nausea, vomiting and balance problems with falls are also described. Brain cells have limited room to swell inside the skull.
Fluid restriction, neuro checks and safe correction
MedlinePlus notes that the first step in managing SIADH is limiting fluid intake, along with treating the cause, such as changing a causative medicine. Explain the restriction to the client and family, spread the allowance across the day, and offer mouth care and ice chips within the limit if permitted. Weigh daily and keep accurate intake and output.
Perform neuro checks and use seizure precautions as local protocol directs. Correction speed matters too: MSD warns that raising sodium too quickly can cause osmotic demyelination syndrome. Report a rapid rise in sodium, new difficulty speaking or swallowing, or weakness after treatment starts. Hypertonic saline, if used, is given under close monitoring per protocol.
Work a hypothetical SIADH scenario
Imagine a hypothetical client with small cell lung cancer whose sodium has fallen, who reports headache and nausea and passes small amounts of dark urine. There is no oedema. Options include encouraging oral fluids to treat apparent dehydration, giving a scheduled diuretic and documenting, or reporting the trend and implementing fluid restriction and neuro checks as ordered.
Reporting and supporting fluid restriction with neuro checks is the correct choice. Dark, scanty urine here reflects ADH-driven concentration, not low volume, so extra fluid would lower sodium further. A diuretic is a prescriber decision and does not replace assessment. The headache and nausea suggest early cerebral effects that need closer monitoring.
Sources and further reading
Syndrome of inappropriate antidiuretic hormone secretion (MedlinePlus Medical Encyclopedia). Excess ADH retains water and dilutes sodium; concentrated urine; causes; symptoms from nausea to seizures; fluid restriction as first step.
Hyponatremia (MSD Manual Professional Edition). SIADH definition and euvolaemic classification; water excess relative to solute; neurologic features from water shift into brain cells; osmotic demyelination with rapid correction.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our endocrine practice questions are the closest set to what this page covers.
Common questions
Why does a client with SIADH not look swollen?
The kidneys respond to the extra volume by excreting sodium, and much of the retained water moves into cells, so little stays in the tissue spaces. That is why SIADH is classed as euvolaemic hyponatraemia even though total body water is increased.
How is SIADH different from diabetes insipidus?
They are opposites. SIADH has too much ADH, concentrated urine and low sodium. Diabetes insipidus has too little ADH effect, large volumes of dilute urine and a tendency toward high sodium and dehydration.
Why must sodium be corrected slowly in SIADH?
Brain cells adapt to low sodium over time. Raising sodium too quickly can draw water out of them and cause osmotic demyelination syndrome. The prescriber sets correction limits; the nurse monitors levels and neuro status closely and reports rapid changes.