On the medicine shelf, hyponatremia usually arrives as an older patient who is confused, has fallen or has just had a seizure, with a sodium in the 110s or 120s and a medication list that includes an SSRI, a thiazide or carbamazepine. The diagnosis is rarely the hard part. The question tests whether you treat severe symptoms first, read serum osmolality, urine osmolality and urine sodium in the right order, and raise the sodium without overshooting.
The algorithm
Work through the steps in order. Severe symptoms jump the queue; everything else follows the labs.
- Screen for severe symptomsVomiting, seizures, deep somnolence, coma or cardiorespiratory distress mean hyponatremic encephalopathy. Give a 3% saline bolus now, aiming for a 4 to 6 mEq/L rise within 1 to 2 hours, then finish the workup. No severe symptoms: go to step 2.
- Confirm it is hypotonicSerum osmolality below 275 mOsm/kg is true hypotonic hyponatremia. Normal osmolality with very high triglycerides or protein is pseudohyponatremia. High osmolality points to glucose or mannitol; add about 2.4 mEq/L to the sodium for every 100 mg/dL of glucose above 100.
- Decide acute or chronicDocumented for under 48 hours is acute. If you cannot tell, treat it as chronic, which makes overcorrection the main danger.
- Read urine osmolality100 mOsm/kg or less means ADH is switched off: primary polydipsia, beer potomania or a tea-and-toast diet. Above 100 means ADH is active, so keep going.
- Examine volume statusEdema, ascites or a raised JVP point to heart failure, cirrhosis, nephrotic syndrome or kidney failure. Dry mucosa and orthostasis point to volume loss. Neither means the patient is euvolemic.
- Check urine sodiumIn a dry patient, 30 mEq/L or less suggests extrarenal loss (vomiting, diarrhea, third spacing), and above 30 suggests renal loss (diuretics, primary adrenal insufficiency, cerebral salt wasting). In a euvolemic patient, above 30 fits SIADH.
- Exclude the SIADH mimicsCheck TSH and cortisol and review the drug list before you call it SIADH. Hypothyroidism, adrenal insufficiency and diuretics, especially thiazides, can produce the same labs.
- Treat by category at a safe rateHypovolemic: isotonic saline. SIADH: fluid restriction first. Hypervolemic: fluid restriction and treat the cause. In chronic hyponatremia, keep the rise to 10 mEq/L or less in the first 24 hours, and 8 or less if the risk of osmotic demyelination is high.
Sources differ on details. The European guideline gives 150 mL of 3% saline over 20 minutes and the US expert panel 100 mL over 10 minutes; both allow up to three boluses. The US panel allows 10 to 12 mEq/L in 24 hours at normal risk, while the more recent European guideline caps the rise at 10 in the first 24 hours and 8 in each 24 hours after that. This post uses the European limits.
Clue in the stem, next best step
| Clue in the stem | What it points to | Next best step |
|---|---|---|
| Seizure, vomiting or coma with a low sodium | Severe hyponatremic encephalopathy | 3% saline bolus now; recheck sodium after each bolus |
| Sodium 128 mEq/L with glucose 600 mg/dL | Hypertonic hyponatremia from glucose (corrects to about 140) | Treat the hyperglycemia, not the sodium |
| Normal serum osmolality, lipemic sample or very high protein | Pseudohyponatremia | No sodium treatment; address the lipid or protein problem |
| Urine osmolality under 100 in a heavy beer drinker or a patient drinking large volumes of water | Low solute intake or primary polydipsia | Limit water, raise dietary solute such as protein and recheck sodium often |
| Vomiting or diarrhea, orthostasis, urine sodium 30 or less | Hypovolemic, extrarenal loss | Isotonic saline or a balanced crystalloid |
| Recently started thiazide, urine sodium above 30 | Diuretic-induced hyponatremia | Stop the thiazide, replace sodium and potassium, do not restart it |
| Euvolemic, urine osmolality above 100, urine sodium above 30, on an SSRI or carbamazepine or with small cell lung cancer | SIADH | Check TSH and cortisol, then restrict fluid |
| Urine osmolality above 500, or urine sodium plus potassium higher than serum sodium | Fluid restriction is likely to fail | Move to urea, or a low-dose loop diuretic with oral salt |
| Edema, ascites or a raised JVP | Hypervolemic hyponatremia (heart failure, cirrhosis, nephrotic syndrome) | Fluid restriction and treat the underlying disease |
| Sodium up 12 mEq/L in 18 hours in a malnourished patient with alcohol use disorder | Overcorrection with high osmotic demyelination risk | Stop active treatment; re-lower with D5W and desmopressin with expert input |
Where students lose points
- Sending a seizing or vomiting patient for a head CT or a full workup first. Severe symptoms are an emergency: bolus first, investigate after.
- Correcting chronic hyponatremia too fast. Osmotic demyelination shows up a few days later as quadriparesis, parkinsonism or a locked-in state. Risk is highest with sodium 105 or lower, hypokalemia, alcohol use disorder, malnutrition or advanced liver disease.
- Choosing 0.9% saline for a euvolemic patient with SIADH. Isotonic saline is for volume depletion. In SIADH it often fails to raise the sodium, and the European guideline treats that failure as a supporting clue for the diagnosis.
- Forgetting the glucose. A sodium of 128 with a glucose of 600 mg/dL corrects to about 140, so the sodium is not the problem.
- Labeling SIADH without checking thyroid and adrenal function and the medication list. Secondary adrenal insufficiency and hypothyroidism are classic look-alikes.
- Missing the rebound. Once volume is restored or a thiazide is stopped, a brisk water diuresis can push the sodium up fast, so recheck it often.
Practice question
A 76-year-old woman is brought to the emergency department after a generalized tonic-clonic seizure at home that stopped on its own after 2 minutes. Her daughter says she has had headaches and worsening confusion for 3 days. Sertraline was started 4 weeks ago for depression, and she takes no diuretics. Temperature is 37.0 C, blood pressure 132/78 mm Hg, pulse 88/min and respirations 16/min. She is drowsy but rousable and vomits twice during the exam. Mucous membranes are moist, there is no edema and the JVP is normal. Labs: sodium 114 mEq/L, potassium 4.1 mEq/L, glucose 96 mg/dL, creatinine 0.8 mg/dL, serum osmolality 238 mOsm/kg, urine osmolality 460 mOsm/kg and urine sodium 58 mEq/L.
Which of the following is the most appropriate next step in management?
- Restrict free water intake to less than 1 L per day
- Infuse 0.9% saline at 125 mL/h
- Give a bolus of 3% saline and recheck the sodium shortly afterward
- Start oral tolvaptan
- Send TSH and a morning cortisol and wait for the results before treating
Show the answer
Answer: C. A seizure, drowsiness and repeated vomiting are severe symptoms, so she needs hypertonic saline now, repeated up to three times if symptoms persist, until the sodium has risen about 4 to 6 mEq/L. The duration is unknown, so treat it as chronic: once symptoms settle, keep the total rise to 10 mEq/L or less in the first 24 hours. Her labs fit SIADH from sertraline: hypotonic serum, urine osmolality above 100, urine sodium above 30 and a euvolemic exam.
Why the others are wrong. A: fluid restriction is first-line for chronic SIADH without severe symptoms, but it raises the sodium far too slowly to treat cerebral edema. B: isotonic saline is for volume depletion; she is euvolemic, and in SIADH it often fails to raise the sodium. D: tolvaptan is started in hospital at a low dose and titrated over days, so it cannot treat a seizure, and the European guideline recommends against vaptans in profound hyponatremia. E: thyroid and adrenal testing is needed before you confirm SIADH, but it should never delay emergency treatment.
60-second recall
- Serum osmolality that confirms hypotonic hyponatremia – below 275 mOsm/kg.
- Urine osmolality that means ADH is suppressed – 100 mOsm/kg or less (primary polydipsia, beer potomania, tea-and-toast diet).
- Urine sodium cutoff in the European algorithm – 30 mEq/L.
- Maximum rise in the first 24 hours of chronic hyponatremia – 10 mEq/L, or 8 if osmotic demyelination risk is high.
- First-line treatment for chronic SIADH, and the second line – fluid restriction; then urea or a low-dose loop diuretic with oral salt.
Keep going: browse every Shelf Prep post, review initial vs best diagnostic tests, talk a case through out loud with MedRise Viva, or find more study resources on MedRise for students.
Sources
- Spasovski G, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia (European Society of Endocrinology, ESICM and ERBP). Eur J Endocrinol, 2014
- Verbalis JG, et al. Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations. Am J Med, 2013
- Adrogué HJ, Tucker BM, Madias NE. Diagnosis and management of hyponatremia: a review. JAMA, 2022
- StatPearls: Hyponatremia. NCBI Bookshelf, updated 2023
MedRise Shelf Prep is educational content for exam preparation only. It is not medical advice and does not replace current guidelines, your school's materials or clinical judgment.
