What is causing my low sodium level?
Ask the doctor
Q. 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?
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 it 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.
Normally, a part of the brain called the hypothalamus makes extra ADH when the blood volume is low. ADH gets released into the bloodstream. ADH acts on the kidney to reabsorb water in an effort to increase blood volume. If a person does not take in any salt and just drinks water, the concentration of sodium in the blood can fall. Once blood volume returns to normal, ADH should turn off and the blood sodium level should correct itself.
However, when a person has SIADH, the hypothalamus continues to make and release excessive ADH despite the person's blood volume being normal. There are many potential reasons for SIADH, including:
- medications, such as thiazide diuretics (hydrochlorothiazide, chlorthalidone), antidepressants such as selective serotonin uptake inhibitors (SSRIs), and opioids
- persistent severe pain
- an underactive thyroid gland (hypothyroidism)
- brain injury
- lung problems (for example, pneumonia or lung tumors).
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About the Author
Howard E. LeWine, MD, Chief Medical Editor, Harvard Health Publishing; Editorial Advisory Board Member, Harvard Health Publishing
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