Low sodium is caused by excess fluid in the body, according to MedicineNet. It may also be due to loss of sodium and body fluid, and it may be caused by kidney failure. Symptoms of low sodium include headache, seizures, confusion or unstable mental state.
A low level of sodium in the blood is referred to as hyponatremia. Sodium is a necessary element for various body processes such as maintenance of fluid balance, blood pressure regulation and normal function of the nervous system, according to MedicineNet. The recommended blood sodium level is 135 to 145 milliequivalents.
Hyponatremia may be caused by certain medications such as water pills, antidepressants and pain medications that cause a person to urinate or perspire more frequently. In addition, congestive heart failure and conditions affecting the kidney and liver may cause fluids to accumulate in the body, which dilute the sodium in the body and decrease the overall level. Severe vomiting and diarrhea may cause the body to lose fluids together with electrolytes, including sodium, according to Mayo Clinic. Drinking too much water dilutes the sodium content of the blood. At the same time, dehydration leads to loss of sodium.
Symptoms of hyponatremia include restlessness, weakness, fatigue and muscle spasms, according to WebMD. Decrease in the sodium level causes water to enter the cells, causing them to swell. This may occur in the brain, leading to a condition known as cerebral edema, where the skull restricts expanding brain cells.