Define SIADH and identify any patient characteristics that may have contributed to the development of SIADH.

SIADH is a group of symptoms that occurs when antidiuretic hormone (ADH, arginine vasopressin) is secreted in the absence of osmotic or physiologic stimuli. These stimuli include: Increased serum osmolality, decreased plasma volume, and hypotension. A decrease in plasma osmolality normally inhibits ADH production and secretion. SIADH is characterized by fluid retention, dilutional hyponatremia, hypochloremia, concentrated urine, and lack of intravascular volume depletion. SIADH is characterized by normal to increased blood volume in normoproteinemia, nonedematous, and hyponatremic patients with normal renal and endocrine function. ADH regulates the body’s water balance. It is synthesized in the hypothalamus and stored in the posterior pituitary gland. When released into the circulation, it acts on the kidney’s distal tubules and collecting ducts, increasing their permeability to water. This decreases urine volume because more water is being reabsorbed and returned to the circulation. It also serves to produce more concentrated urine.