Nursing care
SIADH nursing care: what to assess and what to do first
Written and reviewed by Dana Whitfield, RN, MSN · 6 min read · Updated September 2026
Short answer
SIADH causes the body to retain water while holding onto normal or elevated sodium, so blood volume rises and sodium becomes diluted, not lost. The primary nursing intervention is fluid restriction, not sodium replacement. Correcting sodium too quickly risks osmotic demyelination syndrome, a severe and often permanent brain injury.
Recognising it at the bedside
SIADH, syndrome of inappropriate antidiuretic hormone, means the posterior pituitary or an ectopic source, commonly small cell lung cancer, releases ADH independent of the body's actual fluid status. ADH tells the kidneys to reabsorb water, so the patient retains fluid while continuing to excrete sodium in concentrated urine. The net effect is dilutional hyponatraemia: total body water goes up, sodium concentration goes down, but total body sodium is largely unchanged.
At the bedside this looks deceptively unremarkable early on: weight gain without oedema, concentrated dark urine despite fluid retention, and a patient who is often normotensive or even mildly hypertensive rather than showing the fluid overload signs a nurse might expect. As sodium drops further, neurological symptoms appear: headache, nausea, confusion, lethargy, and in severe cases seizures or coma. The absence of peripheral oedema, in a patient who is clearly retaining water, is the detail most likely to be missed.
Why the classic presentation misleads
Most fluid overload states, heart failure or renal failure, present with visible oedema, crackles, and distended neck veins, and nurses are trained to look for those signs first. SIADH does not produce peripheral oedema because the excess water distributes intracellularly and into the vascular space rather than pooling in dependent tissue, so a patient can be significantly fluid overloaded with clear lungs and no pitting.
This is why the presenting complaint is so often neurological rather than cardiovascular: confusion, lethargy, or a new headache in a patient with a risk factor for SIADH, such as small cell lung cancer, a head injury, or certain medications like SSRIs or carbamazepine, should prompt a sodium check before anything else. A nurse expecting crackles and jugular distension in fluid overload will miss SIADH if they wait for those findings to appear; they usually do not.
Priority nursing actions
Fluid restriction is the primary intervention, typically 800 to 1000 mL per day or as ordered, and it works because it is the mismatch between water intake and impaired water excretion that is driving the dilution. Enforce the restriction consistently, track intake against the limit across the full 24 hours including IV fluids and medication diluents, and involve the patient in choosing how the allowance is spent through the day rather than presenting it as a blanket denial.
Monitor neurological status frequently, since a falling sodium or a patient who is becoming harder to arouse is the earliest warning of progression toward seizure or coma. Weigh the patient daily at the same time, on the same scale, as a sensitive marker of fluid status that will trend before serum sodium sometimes does. Institute seizure precautions if sodium is significantly low. Report worsening confusion, headache, or a sodium trending downward immediately rather than at the next scheduled check, since the neurological trajectory in SIADH can shift quickly.
Labs and diagnostics to expect
Serum sodium below 135 mEq/L confirms hyponatraemia, and in SIADH it is usually a dilutional picture with low serum osmolality alongside an inappropriately concentrated urine osmolality, the hallmark mismatch that distinguishes SIADH from other causes of low sodium. Urine sodium is typically elevated because the kidneys continue excreting sodium even as the patient retains water.
Serial sodium levels guide the pace of treatment and are checked frequently, often every few hours in a hospitalised patient with symptomatic hyponatraemia, because the rate of correction matters as much as the target value. A chest X-ray or CT may be ordered to investigate for an underlying malignancy or pulmonary cause when SIADH is newly diagnosed without an obvious trigger like head trauma or medication. Track osmolality trends alongside sodium, since a rising serum osmolality confirms the fluid restriction is working even before sodium fully normalises.
Complications and their early signs
The complication that changes nursing priorities is not the hyponatraemia itself but the risk of correcting it too quickly. Rapid sodium correction, generally more than 8 to 10 mEq/L in 24 hours, pulls water out of brain cells faster than they can adapt, causing osmotic demyelination syndrome, a form of brain injury that can leave permanent neurological deficits. This is why severe symptomatic hyponatraemia is corrected cautiously even when hypertonic saline is used, and why sodium is rechecked frequently during active correction rather than trusted to a single order.
Watch for early signs that correction is happening too fast: improving mental status followed by new or worsening neurological deficits days later is the classic and alarming pattern of osmotic demyelination, since the injury often manifests after an initial apparent improvement. Uncorrected severe hyponatraemia carries its own risk of seizure, cerebral oedema, and herniation, so the nursing task is holding a middle course: treat the low sodium, but never rush it.
Teaching that changes outcomes
Teach the patient and family that fluid restriction, not extra fluids, is the treatment, since the intuitive instinct when someone is dehydrated-seeming or confused is to offer more to drink, and that instinct is wrong here and can worsen the condition. Explain the daily allowance in concrete terms, such as cups or millilitres per shift, so the restriction is something the patient can actually follow rather than an abstract number.
For a patient going home with chronic SIADH, often from a malignancy or a long-term medication, teach daily weight monitoring at the same time each day as an early warning sign, and teach the specific symptoms, headache, nausea, confusion, or muscle cramps, that mean call the provider rather than wait for the next appointment. If a medication such as an SSRI is the identified cause, explain that any changes to that medication are made by the prescriber, not stopped independently. Reinforce that sodium levels will be monitored on a schedule even after the patient feels well, since the underlying cause driving ADH release may persist.
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 is fluid restriction the main treatment for SIADH, rather than giving sodium?
SIADH causes water retention that dilutes a normal total body sodium, so the problem is excess water, not sodium loss. Restricting fluid intake corrects the dilution directly, while giving sodium without restricting fluids does little because the excess water remains.
What happens if sodium is corrected too quickly in SIADH?
Rapid correction, generally more than 8 to 10 mEq/L in 24 hours, can cause osmotic demyelination syndrome, a serious and often permanent brain injury. Sodium is corrected gradually and monitored frequently for this reason.
Why doesn't a patient with SIADH have oedema despite fluid retention?
The retained water distributes into the vascular space and intracellularly rather than pooling in dependent tissue, so peripheral oedema is typically absent. This is why SIADH is easy to miss if a nurse is looking for the fluid overload signs seen in heart failure.
What is the earliest warning sign that SIADH is worsening?
A change in neurological status, such as new confusion, lethargy, or headache, is often the earliest sign, since it reflects a falling sodium level before more dramatic symptoms appear. Frequent neurological checks are a priority nursing action for this reason.
What conditions commonly cause SIADH?
Small cell lung cancer is a classic cause through ectopic ADH production, along with head injury, certain medications like SSRIs and carbamazepine, and some pulmonary infections. Identifying and treating the underlying cause is necessary alongside fluid restriction for lasting resolution.