Diabetes insipidus, or DI, is a condition characterized by excessive urination and extreme thirst. It is unrelated to diabetes mellitus, which is a disorder of glucose metabolism. DI is usually caused by insufficient production of a pituitary hormone called antidiuretic hormone, or ADH, which tells your kidneys to retain water. If your pituitary gland does not secrete ADH --- or if your kidneys don't respond to it --- your body loses more water than it should. According to "The Merck Manual of Diagnosis and Therapy," sodium restriction is sometimes helpful for treating DI.
Central
Normally, when your blood sodium level increases or when your blood volume falls --- both are signs of dehydration --- your pituitary secretes ADH, which travels through your bloodstream to your kidneys, where it stimulates receptors that increase the reabsorption of water from your urine. This increases your blood volume and normalizes the sodium concentration in your bloodstream. Central DI is caused by a deficiency of pituitary ADH production. Without ADH, your kidneys cannot conserve water even in the face of dehydration.
Nephrogenic
Diabetes insipidus can occur in the face of normal pituitary ADH production. Sometimes, your kidneys become unresponsive to ADH, leading to a condition called nephrogenic diabetes insipidus. Some forms of nephrogenic DI are inherited, but others result from acquired kidney diseases or from damage caused by certain medications. For example, the May 2009 issue of "Nature Reviews Nephrology" outlines the role of lithium --- a drug used to treat bipolar disorders --- in the genesis of diabetes insipidus. Up to 40 percent of patients taking this medication develop nephrogenic DI.
Sodium's Role
If your kidneys waste water even when you are dehydrated, the sodium concentration in your bloodstream tends to rise. High sodium levels --- a condition called hypernatremia --- can lead to confusion, seizures, brain damage, coma and death. Salt restriction helps to prevent hypernatremia in patients with DI. In addition, your kidneys will attempt to unload some sodium if you have DI, and water tends to follow sodium into your urine. This increases your urine volume in a disease where water losses are already inappropriately high. Thus, sodium restriction helps to reduce your urinary volume, thereby conserving water.
Considerations
Diabetes insipidus can result from inadequate pituitary ADH production or from a lack of kidney response to ADH. Genetic disorders, head injuries, kidney disease or certain medications, such as lithium, can cause diabetes insipidus. Urinary water losses in DI can lead to dehydration and sodium imbalances in your tissues. Treatment for DI involves correcting any treatable cause --- stopping lithium, for example --- and administering medications that either mimic ADH or improve your kidneys' response to its effects. Salt restriction is often employed to help maintain normal serum sodium levels and to help reduce urinary output.
References
- "The Merck Manual of Diagnosis and Therapy, 18th Edition: Central Diabetes Insipidus"; Dr. Mark H. Beers; 2006
- "Nature Reviews Nephrology"; Lithium Nephrotoxicity Revisited; J. Grünfeld, B.C. Rossier; May 2009
