Sodium maintains the proper acid-base equilibrium for the proper osmotic balance. Normally it is the most abundant cation (positively charged electrolyte) in the fluid outside of the cell (extra-cellular fluid or ECF). Like all cations, sodium has alkaline properties. Sodium is of greatest importance in osmotic regulation of extra-cellular fluid balance and acid balance, as well as renal, cardiac and adrenal functions. It is needed to maintain the sodium-potassium pump, which transports sodium out of the cell and potassium into the cell. This electrical pump creates a small amount of voltage across the cell membrane and is what nerve conduction depends upon. Given this fact, it is easy to understand why potassium is concentrated in the cells and sodium in the tissues.

Balance with potassium, acid-base influences, and general vascular volume is implied as well. To roughly assess electrolyte balance, add Cl and CO₂ and subtract them from the sodium level. The result of 12-16 indicates good electrolyte balance. The value of sodium is based on its relationship to potassium, chloride, pH, anion gap, osmolality, etc. The urinalysis and specific gravity can give further clues as to the root cause. Combining this information gives an excellent indicator of adrenal cortical function. 

The sodium level by itself is often not useful unless it is severely out of the normal range. It can only be interpreted in relation to the serum potassium and chloride levels.

Optimum Values: 142-145 mmol/L

Sodium levels may be elevated in:

• Water deficit (diabetes insipidus, loss of glucose in the urine due to diabetes mellitus, excessive sweating, inadequate water intake, dehydration).
• Water deficit exceeding sodium deficit (diarrhea, diabetes mellitus, renal disease)
• Excessive intake of sodium (sodium bicarbonate or salt tablet use, sea water, drinking water from water softeners on a prolonged and regular basis.)
• Adrenal Cortex Hyper-function. Aldosterone is produced by the adrenal cortex, with excessive production the body retains sodium from the kidney in exchange for urinating out potassium. This is why you may get relatively low potassium if your adrenal cortex is producing too much aldosterone.
• Diabetes.
• Pyloric Obstruction.
• Congestive Heart Failure. Because the heart is not pumping the blood to the kidneys with enough power, the kidney starts to reclaim as much sodium as possible. Unfortunately this doesn't help the heart because the extra fluid that stays with sodium stresses the heart even further.
• Insufficient Anti-Diuretic Hormone production by the hypothalamic-pituitary complex. As a result, the body loses water but retains the sodium.
• Alcohol intake reduces the sodium lost in the urine.
• Insufficient parathyroid hormone results in reduced loss of sodium in the urine with resultant increased retention of serum sodium.

Sodium may be reduced in:

• Pyloric Spasm.
• Hyperglycemia and Diabetes mellitus lower serum sodium because the sodium is carried out in the urine with the extra glucose.
• Excess Perspiration leads to loss in the sweat.
• Adrenal Cortex Hypo-function. Aldosterone is produced by the adrenal cortex; with insufficient production the body loses sodium in the urine in exchange for potassium. This is why you may get relatively low sodium with relatively elevated serum potassium with insufficient production of aldosterone.
• Excess progesterone blocks the action of aldosterone but insufficient progesterone results in greater loss of sodium in the urine. Both conditions can lead to lowered sodium levels.
• Diarrhea and metabolic alkalosis.
• Renal Dysfunction.
• Syndrome of Inappropriate Anti-Diuretic Hormone production by the hypothalamic-pituitary complex. This hormone causes water retention and sodium excretion. As a result you don't get rid of water and the extra water you hold onto dilutes out the sodium.

Causes of Decreased

Isotonic Hyponatremia (plasma osmolality 280-295)

• Hyperproteinemia
• Hyperlipidemia

Hypertonic Hyponatremia (plasma osmolality >295)

• Hyperglycemia (diabetes)
• Mannitol
• Glycerol

Hypotonic Hyponatremia (plasma osmolality <280)

• Renal losses (diuretics, adrenal cortex hypofunction, renal dysfunction)
• Gastrointestinal (pyloric spasm, diarrhea)
• Edema (CHF, cirrhosis, nephrotic syndrome, hypothyroidism, glucocorticoid deficiency)
• Water intoxication (including tap water enemas, beer)
• Excessive perspiration
• Third spacing (burns, ascites, effusions)
• Excess ADH (SIADH, pain, medications)

*Symptoms depend on the rate that the sodium drops and are primarily neurologic. Check for renal function with a urine sodium done at same time as serum sodium and osmolality. Insufficient adrenal function is not an uncommon finding with a relatively lower sodium and higher potassium.