Sodium is the principal positive ion found in your body fluids. A small amount of sodium is located inside your cells, but it is mostly confined to your extracellular fluids. Along with potassium and chloride, sodium is an important electrolyte that influences the movement of water and nutrients and the generation of electrical currents in your tissues. According to the Linus Pauling Institute at Oregon State University, your body uses several mechanisms to tightly control sodium concentrations. Antidiuretic hormone, or ADH, is one of these mechanisms.
Sodium Functions
In concert with potassium -- the primary positive ion found inside your cells -- sodium is responsible for maintaining a slight difference in electrical charge between the interior and exterior of your cells. This "membrane potential" is needed for transporting fluids and nutrients across cell membranes, and it plays a major role in generating the action potentials that allow your nerves, muscles and heart to function. By adjusting sodium concentrations in its extracellular fluids, your body regulates blood pressure and blood volume. ADH plays an important role in this process.
Antidiuretic Hormone
As its name implies, antidiuretic hormone prevents the loss of water from your body. According to "The Merck Manual of Diagnosis and Therapy," ADH is synthesized in your hypothalamus and transported to your pituitary gland for storage. When sensors in your hypothalamus or blood vessels detect an increase in sodium concentration or a drop in blood volume, ADH is released into your bloodstream. ADH then travels to your kidneys, where it stimulates the reabsorption of water from your urine. This decreases your urine volume, increases your blood volume and "dilutes" the sodium in your bloodstream, thereby reducing sodium concentration.
Inappropriate Secretion
A variety of medical conditions can trigger excessive secretion of ADH. These disorders, collectively called syndromes of inappropriate ADH secretion, or SIADH, can cause excessive water retention and dilution of your blood sodium. Some types of cancer, brain trauma, infections, hormonal disorders, strokes, certain medications, acute psychosis and several other conditions stimulate ADH release even when your blood volume and sodium concentration are normal. SIADH is a relatively common cause of low sodium levels, or hyponatremia. Depending on its severity, hyponatremia can cause confusion, personality changes, irritability, lethargy, seizures, coma and death.
Considerations
Antidiuretic hormone is one of several mechanisms your body uses to maintain optimal sodium concentrations in your extracellular fluids. ADH's principal function is to increase reabsorption of water from your urine, thereby increasing your blood volume and decreasing its sodium concentration. Inappropriate ADH secretion can disrupt your sodium balance and interfere with normal cellular function. When SIADH leads to hyponatremia, water restriction or a medication that interferes with ADH's action is necessary to return your sodium level to normal.
References
- Linus Pauling Institute at Oregon State University: Sodium (Chloride)
- "The Merck Manual of Diagnosis and Therapy, 18th Edition: Posterior Pituitary Function -- Antidiuretic Hormone and Hyponatremia"; Mark H. Beers, M.D., Editor-in-Chief; 2006
