Sodium is a mineral and also functions as an electrolyte in the body.  An electrolyte is defined in the Medline Plus dictionary as: 

1. A nonmetallic electric conductor in which current is carried by the movement of ions.  2a: a substance (as an acid, base, or salt) that when dissolved in a suitable solvent (as water) or when fused becomes an ionic conductor. 2b: any of the ions (as of sodium, potassium, calcium, or bicarbonate) that in a biological fluid regulate or affect most metabolic processes (as the flow of nutrients into and waste products out of cells)—used especially in biology and biochemistry.”

Sodium in the body is controlled by the kidneys and affected by how much you sweat.  It is one of the main electrolytes that determines the amount of fluid in the body and available to the cells.    Sufficient fluid in the body is required to allow electrical impulses between cells.  If there is not sufficient sodium for these electrical impulses, then cells will not function in an optimum manner.  In an extreme situation, improper sodium levels can even lead to fatal consequences.

Sodium combined with chloride becomes table salt. Many foods contain sodium. Too much sodium has been linked to increased blood pressure that can lead to heart problems and even kidney damage.  Sodium can increase water retention and cause edema, or swelling, of the limbs.

TEST RESULTS

The blood sodium level results are normally expressed in milliEquivalents per liter.  MilliEquivalents are one thousandth of a liter.  If the results are between 135 milliEquivalents per liter to 145 milliEquivalents/liter (mEq/L), or in international units, 135 – 145 millimoles/liter (mmol/L), it is considered normal.

If the blood sodium level is higher than 145 milliEquivalents per liter, it is called hypernatremia.  (Hyper = high and natremia = sodium in the blood).  There can be many possible causes but some are:
•    Kidney problems
•    Excessive diarrhea
•    Use of diuretics, either drugs or liquids like sodas, coffee and tea
•    Excessive sweating

If the blood sodium level is less than 135 milliEquivalents per liter, then this is called hyponatremia (Hypo=low and natremia = sodium in the blood).  There can be many possible causes but some are:
•    Endocrine glands not operating properly
•    Poor diet
•    Insufficient electrolytes leading to the inability to retain sufficient water
•    Kidney problems
•    Liver problems