Antidiuretic hormone (ADH) is synthesized and regulated by the hypothalamus. When thirst occurs the body responds by increasing fluid intake and water retention. This helps to increase the volume of water in the body, decreasing osmolality and, therefore, decreasing ADH production. When the body loses an increased amount of fluid, decreasing plasma volume and increasing osmolality, the hypothalamic receptors increase ADH production to retain fluid and create a state of homeostasis (Figure 43-1).

Diabetes insipidus (DI) and syndrome of inappropriate diuretic hormone (SIADH) are two conditions of abnormal ADH secretion that affect fluid and electrolyte balance (Table 43-1). ADH is secreted by the hypothalamus to work on the renal tubules, increasing permeability for water and urea. When an insult to the cranium such as head trauma, tumors, or neurosurgery occurs, ADH secretion may be affected.