Low Sodium Blood Test: What Does It Mean and When Is It Urgent?

A low sodium blood test means the level of sodium in your blood is below the laboratory's reference range. The medical name is hyponatremia. It most often happens because the body is holding too much water, which dilutes the sodium, rather than because of too little salt in food.

Sodium is an electrolyte. It helps control the balance of fluid in the body and supports nerve and muscle function. It is usually measured as part of an electrolyte panel or a basic or comprehensive metabolic panel, so many people find a low result on a routine blood test.

This guide helps you understand the result and prepare for a conversation with your clinician. It cannot tell you the cause of your result or how urgent it is; that needs a clinician who knows your history.

Cleveland Clinic advises going to an emergency department for confusion, seizures, difficulty waking up, a severe headache, or severe vomiting or diarrhoea. If someone with a known low sodium result develops any of these, call local emergency services or go to the nearest emergency department. Do not wait for a routine repeat test.

Speed matters. When sodium falls quickly, symptoms can become severe suddenly. When it falls slowly over a long time, the body has more time to adjust and symptoms may be mild or absent. A clinical guideline from NHS Lothian notes that symptoms usually develop with a rapid change, and that most people tolerate long-standing low sodium well. That is still a reason to follow up, not a reason to ignore it.

Contact your clinician promptly, the same day if possible, if you notice new or worsening symptoms such as:

• headache, nausea or vomiting
• unusual tiredness, low energy or difficulty thinking
• muscle cramps, spasms or weakness
• irritability or restlessness

Laboratories use slightly different ranges, so always compare your result with the reference interval printed on your own report. As examples, MedlinePlus gives a normal range of 136 to 144 mEq/L (the same as mmol/L), and Cleveland Clinic describes sodium below 135 mEq/L as lower than normal.

A result just under the range is often called mild hyponatremia in everyday conversation. A slightly low number is not automatically harmless, and a lower number does not by itself tell you the cause. Clinicians look at the value together with symptoms, previous results, how quickly it changed, other blood results and your medicines.

Cleveland Clinic explains that low sodium is usually caused by too much water diluting sodium; sodium loss is less common. Clinicians often think about it in three groups: the amount of body water rises while sodium stays about the same; both rise but water rises more; or both fall but sodium falls more. Causes listed by MedlinePlus and Cleveland Clinic include:

• vomiting, diarrhoea or heavy sweating
• heart failure, kidney disease, liver cirrhosis or nephrotic syndrome, which can make the body hold extra fluid
• SIADH (syndrome of inappropriate antidiuretic hormone), where the body does not get rid of water properly; some cancers, brain and lung conditions can cause it
• an underactive thyroid (hypothyroidism) or Addison disease
• heavy beer or alcohol intake, or malnutrition
• some recreational drugs, including MDMA (ecstasy)

Yes. Diuretics, often called water pills, are a well-known cause. Cleveland Clinic also lists SSRI antidepressants, and MedlinePlus notes that some antidepressants and many over-the-counter pain relievers can affect sodium.

If you take any of these, do not stop or change them on your own. Instead, tell the clinician exactly what you take, including the start date of anything new and any recent dose change, and ask whether a medicine could be contributing. A pharmacist can also help you put together an accurate list.

It can, but Cleveland Clinic says this is rare when the kidneys work normally. The risk is higher during long endurance events such as marathons, when people sweat heavily and drink large amounts without replacing electrolytes. MedlinePlus and Cleveland Clinic suggest drinks that contain electrolytes during vigorous activity and sporting events.

If you have been told to drink more or less fluid for a health condition, follow that plan and ask before changing it. Limiting fluid is one of the treatments clinicians sometimes use, but it is not right for every cause and should be guided by your care team.

According to Cleveland Clinic, assessment may include more blood tests for sodium, potassium, hormones, and kidney and liver function, along with urine tests. Depending on symptoms, imaging such as a chest X-ray or CT scan may be arranged. Treatment depends on the cause and can range from treating the underlying condition to limiting water intake or fluids given through a drip (IV).

Ask how and when you will get the repeat result, and who to contact if symptoms change before then.

Bring the full report, earlier sodium results if you have them, and an up-to-date medicine list. These questions can help:

• How low is my sodium compared with this lab's range, and how does it compare with my earlier results?
• Do you think this came on quickly or slowly, and does that change what we do?
• Could any of my medicines, including over-the-counter pain relievers, be contributing?
• Should I change how much I drink, or keep to my usual habits until we know more?
• Which other tests do I need, and when will my sodium be rechecked?
• Which symptoms mean I should go to an emergency department instead of waiting?

On SameCase, patients and verified doctors share de-identified cases. Reading how others with a low sodium result described their medicines, symptoms and follow-up tests can help you notice details worth mentioning and questions you had not thought to ask.

Another person's case cannot explain your result or tell you what to do. We suggest using what you read only to prepare for the conversation with your own clinician, and seeking urgent care for any warning symptom above.