Low sodium levels in the blood occur when there is a lower than normal concentration of sodium in the blood. This is a condition often referred to as hyponatremia. Sodium is an electrolyte that helps maintain blood pressure and helps regulate nerve and muscle function. Low sodium levels can cause symptoms including confusion, lethargy, and cramping, which are usually brought on by other medical conditions or illnesses. Low sodium levels in the blood can occur due to a number of causes and conditions that can be detected.