Symptomatic Hyponatremia at 116 mmol/L in a Patient on a Thiazide
A 74-year-old woman is admitted with 2 days of confusion and falls. She started hydrochlorothiazide 25 mg daily 3 weeks ago for hypertension and takes no other new medications. She is not orthostatic; JVP is normal, mucous membranes moist, no edema. Labs: sodium 116 mmol/L, serum osmolality 246 mOsm/kg, glucose 96 mg/dL, urine osmolality 486 mOsm/kg, urine sodium 62 mmol/L, uric acid 2.4 mg/dL, TSH 1.8 mIU/L, creatinine 0.9 mg/dL. She is drowsy but follows simple commands, has no seizure and no focal deficit. The family says she has been slowing down for about a week.
1. What is your initial treatment, how fast would you correct, and what is your 24-hour target?
Read the worked answer
This is chronic (at least 1 week), moderately symptomatic hyponatremia without seizure, coma, or respiratory compromise - the category in which you correct cautiously but still actively.
Immediate: give 3% hypertonic saline 100–150 mL intravenously over 20 minutes, with the goal of a 4–6 mmol/L rise (to roughly 120–122) to relieve symptoms. Then stop the bolus and reassess; do not continue chasing a number. Recheck sodium 2–4 hours after each intervention and then every 4–6 hours while correcting.
Ceiling: no more than 8 mmol/L in the first 24 hours (and no more than 18 mmol/L in 48 hours). Lower the ceiling to 4–6 mmol/L per 24 hours if additional risk factors for osmotic demyelination are present - alcoholism, malnutrition, hypokalemia, liver disease. Her chronicity and age put her at risk, so err on the low side.
Stop the hydrochlorothiazide and do not restart it.
Note that the urine osmolality of 486 with a urine sodium of 62 will not fall promptly after the thiazide is withdrawn, so free-water excretion may be poor and the sodium can rise faster than intended once the stimulus (volume depletion, if any) is corrected.
2. Twelve hours later the sodium is 130 mmol/L. What do you do and why?
Read the worked answer
That is overcorrection - a 14 mmol/L rise in 12 hours, already exceeding the 24-hour ceiling. The risk is osmotic demyelination, which can present days later as dysarthria, quadriparesis, pseudobulbar palsy, or locked-in state.
Act now, not when symptoms appear:
- Re-lower the sodium with 5% dextrose in water, typically 3–10 mL/kg over 1–2 hours, with rechecks every 1–2 hours. A common approach is to target a re-lowering to no more than 8 mmol/L above admission in the first 24 hours - here, back toward about 124.
- Add desmopressin 1–2 mcg (intravenous or subcutaneous) to stop ongoing free-water loss and prevent further rise. Desmopressin plus D5W is the standard rescue, and many centers use an upfront desmopressin strategy in high-risk patients.
- Involve nephrology or critical care and consider a higher level of monitoring.
- Look for the driver of the rapid rise - a brisk water diuresis after removing the offending thiazide, correction of hypovolemia, or a potassium shift.
Do not give more hypertonic saline. Recheck sodium every 1–2 hours until it is stably within the desired range.
3. What is the likely mechanism, and what else must be excluded before settling on it?
Read the worked answer
The picture is thiazide-associated hyponatremia with an SIADH-like physiology: euvolemic on exam, urine osmolality above 100 (486), urine sodium above 30, low serum osmolality, low uric acid (2.4), and normal thyroid function. Thiazides cause this through natriuresis plus impaired free-water excretion and, in some patients, a genuine SIADH-like effect; it typically occurs in the first few weeks of therapy, and elderly women are at highest risk.
Before finalizing:
- Adrenal insufficiency - check a morning cortisol; it mimics SIADH and is dangerous to miss. Hyponatremia with hyperkalemia or hypoglycemia should push this up the list.
- Medications - SSRIs, carbamazepine, tramadol, desmopressin, PPIs, and recently stopped diuretics all matter.
- Volume status reassessment - the exam can be misleading; a low BUN and a urine sodium that is high could reflect diuretic effect rather than true euvolemia.
- Imaging - a chest radiograph and brain imaging if there is any focal finding; small-cell lung cancer and other tumors can present as SIADH.
- Pseudohyponatremia - excluded here by the low measured serum osmolality.
Long term: avoid thiazides and favor an alternative antihypertensive; recheck sodium after any new diuretic exposure.