Why? To determine the causes of excess or deficient antidiuretic hormone (ADH or vasopressin) associated with the control of water and sodium levels in the blood: diabetes insipidus, syndrome of inappropriate antidiuretic hormone secretion (SIADH).
What is measured? ADH is a hormone produced in the hypothalamus and stored in the pituitary gland (two structures at the base of the brain). The role of ADH is to control water loss through the kidneys (diuresis) to maintain normal sodium levels in the blood. When sodium levels rise, ADH is secreted to limit water loss through the kidneys, resulting in very dark urine. In the very rare diabetes insipidus (lack of ADH secretion or effect), the kidneys fail to conserve water, and the person produces very abundant (polyuria) and diluted urine that they must compensate for by drinking large amounts of water (polydipsia). In the slightly more common SIADH, the opposite is true, with excessive ADH, resulting in increased water in the blood with low sodium levels (hyponatremia). Symptoms of hyponatremia include nausea, vomiting, weakness, confusion, etc.