Low sodium (hyponatraemia): causes and when to test

Almost everyone who sees a low sodium on their result thinks the same thing: I need to eat more salt. In most cases that is wrong. A low sodium is usually a water problem, not a salt problem.

Your sodium value measures the concentration of sodium in your blood, so salt divided by water. If the water rises, the concentration falls, even though the amount of salt has not changed. Dilution, not loss.

That insight changes the whole question. Not "where did my salt go", but "where is all that water coming from".

What is a normal sodium value?

Laboratories generally use roughly 135 to 145 mmol/l for serum sodium. Below 135 is called hyponatraemia, above 145 hypernatraemia. The range is narrow, because your body steers it tightly through thirst and through your kidneys.

Sodium is the main substance outside your cells and therefore decides how much water sits where. If the sodium concentration changes, water moves with it, including into your brain cells. That is exactly why a rapidly falling sodium produces neurological complaints (Adrogue 2000, PMID 10824078).

What is hyponatraemia and how do you notice it?

Hyponatraemia is a sodium value below roughly 135 mmol/l. A mild drop often gives no complaints or vague ones: headache, nausea, tiredness or poor concentration. With a faster or deeper drop, confusion, drowsiness and in severe cases seizures can occur, because water is pulled into brain cells.

How fast it happens weighs more heavily than how low the number is. Someone who drops to 128 mmol/l in two days can be more unwell than someone who has been sitting there stably for months (Hoorn 2017, PMID 28174217).

In older people, hyponatraemia is relatively often a side effect of medication. Diuretics and certain antidepressants top that list.

Can you drink too much water?

Yes, and this is the cause that strikes me most in athletes. Anyone who drinks large volumes of water during a long effort can dilute their sodium below the safe range. This is called exercise-associated hyponatraemia and it is well described in endurance runners (Hew-Butler 2015, PMID 26227507).

Picture two runners in the same marathon, one finishing with a sodium of 139 mmol/l and the other with 128 mmol/l. The first simply drank to thirst. The second took a full cup at every station as a precaution, and finished nauseous and with a headache. Same distance, opposite problem.

That is the practical point: drinking more is not automatically better. My own rule of thumb is simple. Drink to thirst, not to a schedule.

What does a high sodium mean?

A sodium above roughly 145 mmol/l is called hypernatraemia, and that too is mostly about water. Your blood is then too concentrated, usually because too little fluid comes in or too much is lost. Eating salt is rarely the cause.

You therefore see it more often in people who do not feel thirst well or cannot drink unaided, for instance older people during a heatwave. Persistent fever, vomiting and diarrhoea can lift the value too. The complaints resemble dehydration: thirst, drowsiness, confusion.

To be clear: this is not a value to act on by yourself. An abnormal sodium belongs in a conversation with your GP, in either direction.

Does a low sodium mean you should eat more salt?

Usually not, and this is the biggest misconception around this value. Because a low sodium generally arises from too much water or from medication, extra salt does not solve the underlying problem. What is needed depends entirely on the cause, and your GP establishes that.

The Dutch Voedingscentrum also points out that the average person here takes in too much salt rather than too little, mainly through processed products. Eating more salt on your own initiative is therefore rarely the right conclusion from a low result.

To understand how salt and your blood values connect, read salt and your blood values.

When do you have your sodium measured?

Measuring is useful with complaints that fit a disturbed fluid balance, or with medication that affects it. Think of unexplained confusion, persistent headache with nausea, use of diuretics or antidepressants, or heavy drinking during endurance sport. Without symptoms, a stand-alone sodium test adds little.

Sodium is almost always read together with potassium and kidney values, because those three hang together. You can have sodium and potassium measured in an extended health checkup at Vitalcheck, without a referral.

For the wider picture, read the pillar on minerals and trace elements, and for sodium's counterweight, your potassium value. An abnormal sodium belongs with your GP, certainly if you have symptoms.

References

• Adrogue HJ, Madias NE. Hyponatremia. N Engl J Med. 2000. PMID 10824078.
• Hoorn EJ, Zietse R. Diagnosis and Treatment of Hyponatremia. J Am Soc Nephrol. 2017. PMID 28174217.
• Hew-Butler T, et al. Statement of the 3rd International Exercise-Associated Hyponatremia Consensus Development Conference. Br J Sports Med. 2015. PMID 26227507.
• Voedingscentrum. Salt.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.