Sodium is an electrolyte that helps regulate blood pressure and fluid levels in your body; it also plays a role in the normal function of nerves and muscles. A blood sodium level below 135 mmol/L is considered low. Headache, nausea, weakness, confusion or lack of energy are typical symptoms of low sodium, also called hyponatremia. However, you may not experience any symptoms. Hyponatremia is caused by several diseases that affect water and sodium levels.
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Hypervolemic hyponatremia is a condition when a low sodium level in your blood is due to an increase of water and sodium -- but the water increase is greater and dilutes the sodium. The most common causes of hypervolemic hyponatremia are heart, kidney and liver failure. During congestive heart failure, the heart cannot pump a sufficient amount of blood. This causes fluid accumulation in the body. Cirrhosis is characterized by scarring of the liver and poor liver function, which also causes fluid accumulation. Kidney disease that results in protein can also lead to excess fluid accumulation and hyponatremia.
In euvolemic hyponatremia, the amount of sodium in your body stays the same while the amount of water increases. Euvolemic hyponatremia is the most common form of low sodium diagnosed in the hospital setting. This often due to infusion of large volumes of certain types of IV fluids. A condition called the syndrome of inappropriate antidiuretic hormone secretion, or SIADH, can also lead to euvolemic hyponatremia. In SIADH, excess amounts of the hormone ADH are produced. ADH regulates body water. Excess ADH causes the kidneys to retain too much water. Certain types of cancer, lung infections, and brain disorders can cause SIADH. Low thyroid function, or hypothyroidism, may also lead to euvolemic hyponatremia in some people.
When both sodium and water are lost from your body, but sodium is lost to a greater extent, hypovolemic hyponatremia results. This condition can develop with certain types of chronic kidney disease: sodium that would normally be retained by the kidneys is lost in the urine. With Addison disease, fluid-regulating hormones are produced in insufficient amounts, causing loss of sodium and water, to a lesser extent. Hyponatremia may also occur when excessive water consumption occurs during strenuous exercise such as running a marathon. For example, when sweating, you lose sodium. If you drink excessively large volumes of water, the remaining sodium in your blood can be diluted. Diarrhea or vomiting can also lead to hyponatremia, due to the loss of fluids and sodium. Head injuries, bleeding in the brain and certain types of brain surgery may lead to hypovolemic hyponatremia as well.
Certain medications and drugs can alter sodium levels in your body. Diuretic medications commonly cause hyponatremia, because they stimulate your kidneys to excrete sodium in the urine. Other medications, such as certain antidepressants and pain medications, can lead to excess ADH hormone production. SIADH can also develop with use of the street drug MDMA, commonly called Ecstasy.
- Clinical Endocrinology: Hyponatremia
- Postgraduate Medical Journal: Hyponatraemia in Clinical Practice
- Tietz Textbook of Clinical Chemistry and Molecular Diagnostics; Carl Burtis, et al.