Overhydration occurs when the body takes in more water than it loses. The result is too much water and not enough sodium. Thus, overhydration generally results in low sodium levels in the blood (hyponatremia. See  Minerals and Electrolytes: Hyponatremia).

Usually, drinking large amounts of water does not cause overhydration if the pituitary gland, kidneys, liver, and heart are functioning normally. To exceed the body's ability to excrete water, an adult with normal kidney function would have to drink more than 2 gallons of water a day on a regular basis.

Overhydration is much more common among people whose kidneys do not excrete urine normally—for example, among people with a disorder of the heart, kidneys, or liver. Overhydration may also result from syndrome of inappropriate secretion of antidiuretic hormone (SIADH). In this syndrome, the pituitary gland secretes too much antidiuretic hormone, stimulating the kidneys to conserve water (see When the Body Has Too Much Antidiuretic Hormone).

Brain cells are particularly susceptible to overhydration (as well as dehydration). When overhydration occurs slowly, brain cells have time to adapt, so few symptoms occur. When overhydration occurs quickly, confusion, seizures, or coma may develop.

Doctors try to distinguish between overhydration and excess blood volume. With overhydration and normal blood volume, the excess water usually moves into the cells, and tissue swelling (edema) does not occur. With overhydration and excess blood volume, an excess amount of sodium prevents the excess water from moving into the cells; instead, the excess water accumulates around the cells, resulting in edema in the chest, abdomen, and lower legs.

Treatment

Regardless of the cause of overhydration, fluid intake usually must be restricted (but only as advised by a doctor). Drinking less than a quart of fluids a day usually results in improvement over several days. If overhydration occurs because of heart, liver, or kidney disease, restricting the intake of sodium (sodium causes the body to retain water) is also helpful.

Sometimes doctors prescribe a diuretic to increase urine excretion. In general, diuretics are more useful when overhydration is accompanied by excess blood volume.