A Mildly Low Sodium with Normal Kidney Function
A low sodium is almost never a salt problem. It is a water problem: there is too much water for the salt you have, and the concentration falls. That reframing explains why eating more salt is not the answer, and why the medicines that cause it are ones you would never associate with sodium.
The pattern on your report
- Sodium Low · mild Key
- Creatinine Normal Key
- eGFR Normal Key
- Potassium Normal Key
Printed as: Creatinine in umol/Lor mg/dLeGFR in mL/min/1.73m2Potassium in mmol/Lor mEq/LSodium in mmol/Lor mEq/L— Identical figures, so 132 mmol/L is 132 mEq/L.
Why the numbers look like this
Your body regulates sodium concentration, not sodium quantity, and it does that by adjusting how much water it keeps. The hormone in charge is vasopressin, which tells the kidney to hold water back. When vasopressin is being released for a reason unrelated to your fluid state, water is retained, the sodium in your blood is diluted, and the number falls with your total body sodium unchanged.
Plenty of ordinary things release vasopressin inappropriately: pain, nausea, several common drug classes, lung and brain conditions, and some cancers. The other route is simpler. Drink more water than the kidney can excrete, as endurance athletes occasionally do, and the same dilution happens without any hormone involved.
Not being flagged is not the same as normal
Most reports draw the line at about 135 mmol/L, and a result a few points under it in someone who feels entirely well is common. Two things matter far more than the number itself. The first is how fast it fell, because the brain adapts to a slow decline and tolerates it, while a rapid one causes symptoms at a much higher sodium. The second is whether there are symptoms at all, because a person with headache, nausea or confusion needs attention regardless of how mild the figure looks.
What else on the report can hide this
This is one of the few results that genuinely cannot be interpreted from the blood alone. The paired tests that sort it out are serum and urine osmolality with a urine sodium, and they need to be taken before any fluid is given, because treatment changes them within hours. Getting those samples first is the most useful thing that can happen on the day.
Thyroid function and a morning cortisol belong in the same draw. Underactive thyroid and adrenal insufficiency both cause a low sodium, both are treatable, and both are commonly missed because the sodium gets attributed to a diuretic instead. Check the glucose too: a very high glucose pulls water into the bloodstream and lowers sodium by dilution, which is not the same problem and does not need the same treatment.
What usually causes it
Listed from most to least common — not from most to least serious.
- Very common
Thiazide diuretics
Bendroflumethiazide, indapamide and hydrochlorothiazide, usually within the first weeks of starting. The commonest drug cause by a wide margin, and more likely in older adults and in women.
- Very common
SSRIs and other antidepressants
Cause vasopressin release, with the risk highest in the first month and in older adults. Frequently missed because the tiredness and low mood it produces look like the depression being treated.
- Common
Vasopressin released by illness or pain
Pneumonia, any acute illness, pain, nausea, or recent surgery. Resolves as the underlying problem does.
- Common
Drinking more water than the kidney can excrete, or eating too little solute
Endurance events, psychogenic polydipsia, or a fixed belief about how much water is healthy; and the low-solute version, heavy beer drinking with little food or a tea-and-toast diet, where the kidney runs out of salt to excrete water with. The urine is dilute rather than concentrated, which distinguishes it from every other cause here.
- Common
Other medicines
Proton pump inhibitors, carbamazepine, some antipsychotics, and MDMA. The timing against starting the drug is the clue.
- Uncommon
Hypothyroidism or adrenal insufficiency
Both treatable, both easy to miss. Adrenal insufficiency classically pairs the low sodium with a high potassium, which is the combination worth noticing on this panel.
- Uncommon
Heart, liver or kidney failure
Total body water is increased and the person is visibly fluid-overloaded, with swelling or ascites. The sodium is low despite an excess of both salt and water.
- Uncommon
A lung or brain condition, or a cancer
Small cell lung cancer is the classic source of inappropriate vasopressin. Considered when the common causes have been excluded and the sodium stays low.
What is usually checked next
- Serum and urine osmolality with a urine sodium, taken before any fluid is given Separates dilution from vasopressin excess from salt loss. Give fluid first and these become uninterpretable, which is why the timing matters more than the tests.
- A medication review covering the last three months Two drug classes account for a large share of these, and stopping or switching is often all the treatment needed.
- TSH and a morning cortisol Covers the two endocrine causes, both of which are treatable and both of which get attributed to a diuretic instead.
- Glucose and triglycerides A very high glucose lowers sodium by dilution, and very high triglycerides can produce a falsely low reading altogether.
- Repeat with an assessment of fluid state Whether you are dry, normal or overloaded splits the causes into three groups, and it is a clinical judgment rather than a test.
When to seek care sooner
- Emergency Confusion, drowsiness, a seizure, or difficulty waking
- Emergency Severe headache with vomiting
- Emergency A sodium that has fallen sharply since a recent test
- Same day New weakness, unsteadiness, or repeated falls
- Same day A low sodium with a high potassium and low blood pressure
- Soon A sodium that stays low after the obvious drugs have been stopped
Questions worth bringing to your appointment
- Could my diuretic or antidepressant be causing this?
- Can we get urine and serum osmolality before any treatment is started?
- Have my thyroid and cortisol been checked?
- How does this compare with my sodium on previous tests, and how fast has it fallen?
- How much water should I actually be drinking?