Syndrome of inappropriate antidiuretic hormone secretion, or SIADH, is a complication of many conditions including certain types of cancer, infections and use of some pharmaceuticals. In most cases, the pituitary overproduces antidiuretic hormone (ADH) which discourages urination. This results in a fluid buildup in blood, but not the rest of the tissues. The patient's level of sodium starts to drop even though urination may still continue.
In the normal human body, certain feedback mechanisms prevent the pituitary from producing ADH when the patient is well hydrated. This encourages urination. However, in a patient with SIADH, the hormone is produced continuously no matter what the level of hydration.
SIADH can be diagnosed by blood tests which will show a combination of low sodium levels and high blood volume with low relative red blood cell volume. The patient will show no sign of edema or high blood pressure usually associated with other conditions that cause fluid buildup, such as kidney failure.
Once SIADH is diagnosed, fluid intake should be severely restricted. SIADH is caused by many underlying conditions and treating the underlying condition will usually relieve the condition.