Sodium, or natrium in Latin, is an essential mineral, which is necessary for the maintenance of blood volume and pressure, as well as muscle and nerve functioning. Approximately 90 percent of sodium is found in the extracellular fluid, or ECF, which includes blood and fluid surrounding the cells. Mild to moderate hyponatremia, or sodium blood concentration below 135 mEq/L, is common in hospitalized patients, and is usually asymptomatic. However, severe hyponatremia can lead to brain edema, severe neurological deficits, or death.
The most common type of hyponatremia is the dilutional hyponatremia, which results from water retention and impaired urine output. In dilutional hyponatremia, normal ECF volume is used to diagnose euvolemic or normovolemic hyponatremia, such as seen in the syndrome of inappropriate antidiuretic hormone release, or SIADH, hypothyroidism, glucocorticoid deficiency, increased fluid intake, and low dietary sodium.
Syndrome of Inappropriate ADH release
Urinary excretion of water requires suppression of the antidiuretic hormone, or ADH, which is released by the pituitary, and acts on the kidneys to conserve water. If the ADH is not suppressed, normal water intake results in water retention and diluted ECF sodium concentration. This is the most common cause of euvolemic hyponatremia. According to a 2004 article in the "American Family Medicine" journal, the common causes of SIADH are trauma, stroke, tumors, pneumonia, infection, illicit drugs, and chemotherapeutic and diabetic medications.
Hypothyroidism
Hypothyroidism is a condition in which thyroid gland produces very low levels of the thyroid hormone. It is the result of thyroid damage and dysfunction, or inadequate thyroid-stimulating hormone production by the pituitary gland. Markedly decreased urine output with hyponatremia is only seen in patients with severe hypothyroidism, reports a 2007 article in the "American Journal of Medicine."
Glucocorticoid Deficiency
Glucocorticoids are a group of steroid hormones, which are produced by the adrenal gland, and play an important role in metabolism and immunity. Glucocorticoid deficiency occurs as a result of adrenal gland damage or pituitary dysfunction in which adrenocorticotropic hormone, or ACTH, secretion is impaired. Normally, ACTH stimulates the adrenal gland to produce glucocorticoids, such as cortisol. In glucocorticoid deficiency, concentration of ADH in the blood is increased, and water excretion is impaired, states a 2003 article in the "Journal of Intensive Care Medicine."
Increased Fluid Intake
Primary polydipsia is a disorder of excessive water intake due to an increase in thirst sensation. It is most often see in psychiatric illness treated with antipsychotic drugs, which cause dry mouth. It is also found in patients with hypothalamic lesions, where the thirst center resides, and in over hydration during exercise, reports UpToDate.
Low Dietary Sodium
According to a 2007 article in "Essentials of Pathophysiology", a healthy body maintains a steady concentration of sodium by increasing or decreasing urine production and excretion of sodium. Consuming a low-protein diet, combined with high water intake, leads to decreased urine excretion and hyponatremia. In general, alcohol consumption causes increased urine output. However, chronic consumption of beer, which is high in carbohydrates and low in sodium or protein, results in decreased urine output and hyponatremia.
References
- "American Family Physician"; Management of Hyponatremia; Kian Peng Goh M.R.C.P.; May 2004
- "American Journal of Medicine"; Hyponatremia Treatment Guidelines 2007: Expert Panel Recommendations; Joseph Verbalis M.D. et al.; 2007
- "Journal of Intensive Care Medicine"; Hyponatremia in Critically Ill Patients; T.J. Vachharajani et al.; 2003
- UpToDate; Causes of Hyponatremia; Richard Sterns M.D.; May 2010
- "Essentials of Pathophysiology; Carol Porth, M.D.; 2007
