My patient had a low sodium level in his blood. I puzzled over the cause. He had a habit of drinking ten glasses of water a day, and I knew it was possible to dilute the blood with too much water and lower the sodium level. At the same time he took diuretics to help control his blood pressure, and though diuretics remove free water from the body, they take sodium along at the same time.  In fact, diuretic use, especially though not exclusively hydrochlorothiazide, are one of the major causes of low sodium counts.

So I restricted the amount of water he drank and stopped his diuretic, and his sodium level quickly returned to normal.

The amount of sodium in your blood is directly tied to the volume of fluid.  Sodium draws water to it and helps keep the amount of fluid in your arteries and veins at the proper level. There is an old saying in medical school, that water follows sodium in the body. When the sodium level in your blood is too low (hyponatremia), the first question your internist must answer is whether the volume of fluid is low, normal or elevated.

If you are dehydrated and have a low sodium, the first course of treatment is to replace the lost volume, as with my patient, and the low sodium usually corrects.

If your blood volume is normal and your sodium is low, medications may be the culprit (including anti-depressants, anti-seizure medications, or the recreational drug Ecstasy). There is also an important hormone called anti-diuretic hormone which causes the kidney to retain fluid. Too much of this hormone leads to too much fluid and a low sodium. Causes include lung cancer and problems with the nervous system.

If you have low sodium and your blood volume is normal, the initial treatment is often to fluid restrict the patient to under two liters of fluid intake (or less) per day, in order to counter the dilution of drinking water. At the same time the underlying cause needs to be addressed, whether it is hormonal or a side effect of medication.

The third category is when you have  low blood sodium and are volume overloaded, either as a result of a failing heart, failing liver, or failing kidneys. In this case the initial treatment is diuretics, to restore the volume to normal, which often returns the sodium to normal at the same time.

Low sodium levels cause confusion, fatigue, muscle aches, headaches, and nausea. Be on the lookout for these symptoms. Sudden drop in sodium can also lead to seizures, and if not corrected, to death.

Monitoring blood sodium is a tricky but crucial part of medicine. It is one of the rare times when causes and treatments can be exactly opposite. Diuretics can lead to low sodium when the blood volume is low, but treat it when the volume is high. Too little fluids can cause low sodium if you are dehydrated, but treat a low sodium if you aren’t dehydrated.

With the summer months coming up, low sodium and hyponatremia are sure to be more of a problem. Stay well hydrated in the heat with electrolyte solutions, but at the same time make sure you have regular checkups to make sure your sodium level is normal.

Dr. Marc Siegel, a practicing internist, joined FOX News Channel (FNC) as a contributor in 2008..