Hyponatremia

Hyponatremia (American English) or hyponatraemia (British English) is low sodium concentration in the blood. Normal serum sodium levels are between approximately 135 and 145 mEq/liter (135 - 145 mmol/L). Hyponatremia is generally defined as a serum sodium level of less than 135 mEq/L and is considered severe when the serum sodium level is below 125 mEq/L.

Too little sodium in the diet alone is very rarely the cause of hyponatremia, although it can promote hyponatremia indirectly and has been associated with MDMA-induced hyponatremia. Sodium loss can lead to a state of low blood volume, which signals the release of anti-diuretic hormone (ADH). ADH release leads to water retention and dilution of the blood resulting in a low sodium concentration.

Many conditions including congestive heart failure, liver failure, kidney failure and pneumonia are commonly associated with a low sodium concentration in the blood. This state can also be caused by overhydration from drinking too much water because of excess thirst (polydipsia). Exercise-associated hyponatremia (EAH) is common in marathon runners and participants of other endurance events. 13% of the athletes who finished the 2002 Boston Marathon were in a hyponatremic state, that is, the salt levels in their blood had fallen below usual levels.