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Question : My blood tests indicate show that I have a low sodium level. It was 127 mEq/L on the most recent test and 130 mEq/L the year before. What can cause this?
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The Trusted Source
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Howard LeWine, M.D.

Howard LeWine, M.D., is chief editor of Internet Publishing, Harvard Health Publications. He is a clinical instructor of medicine at Harvard Medical School and Brigham and Women's Hospital. Dr. LeWine has been a primary care internist and teacher of internal medicine since 1978.

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May 04, 2011
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A:

The normal level of sodium in the blood is between 135 mEq/L (milliequivalents per liter) and 145 mEq/L. A level below 135 is low and is called hyponatremia.

Because there are many causes of hyponatremia, I first try to determine whether some other substance is taking up space in the blood volume.

For example, a high blood sugar will make a sodium blood test appear low, even though the true concentration is normal. This is called pseudohyponatremia. High protein or high triglyceride levels in the blood can cause the same effect.

Next, I examine the person to determine his or her fluid status:

  • Is the person dehydrated?
  • Does the person have excess body fluid? (Evidence can include swollen legs or a swollen belly)
  • Does the person's fluid status appear to be normal?

If you were dehydrated the day of your blood test and drank mostly water in the day or two before your test, your body would hold on to as much fluid as possible, even if meant lowering the blood sodium level.

Because your sodium level was low on two occasions, and you would have to be very dehydrated to get a sodium level as low as 127, I suspect there is some other cause.

If you have swelling in your legs or abdomen from excess fluid, the low sodium level could be related to liver problems (such as cirrhosis) or heart failure.

Assuming that your fluid status is "normal," (i.e. you are not dehydrated and have no swelling), you likely have a condition called SIADH. This stands for "syndrome of inappropriate antidiuretic hormone (ADH) secretion."

The kidneys produce ADH when they detect blood volume is lower than normal. ADH tells the kidneys to reabsorb water from the urine, but to let sodium go. This means the blood sodium level falls. When blood volume is normal, ADH should turn off. In people with SIADH, the blood volume is normal, but the kidneys keep making ADH.

There are multiple causes of SIADH, such as:

  • Medications, especially thiazide diuretics
  • Persistent severe pain
  • An underactive thyroid gland (hypothyroidism)
  • Brain injury
  • Lung problems (e.g. pneumonia and lung tumors)

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