10 - Posterior Pituitary

Abstract

Osmolality of body fluids represents one of the most highly regulated homeostatic processes in mammals, with maintenance of plasma osmolality within 1% to 2% tolerances. This is accomplished via coordinated regulation of vasopressin secretion from the posterior pituitary, vasopressin activation of its V2 receptors in the kidneys, and thirst from higher brain centers. Although the regulation of vasopressin secretion and thirst represents a simple but elegant system to tightly control water balance, abnormal functioning of these complex systems can lead to the development of pathologic states. Disorders of body fluids are among the most commonly encountered problems in clinical medicine, in large part because many different disease states can disrupt the finely balanced mechanisms that control the intake and output of both water and solute. Similar to other endocrine glands, disorders of the posterior pituitary, or neurohypophysis, can be broadly divided into disorders of hypofunction, where vasopressin secretion or V2 receptor–mediated effects in the kidney are deficient, and disorders of hyperfunction, where vasopressin secretion or V2 receptor–mediated effects in the kidney are elevated or osmotically inappropriate. The former produces diabetes insipidus and the latter the syndrome of inappropriate antidiuresis. This chapter will cover the physiology of vasopressin and oxytocin synthesis, secretion, and end-organ effects, followed by detailed descriptions of the pathophysiology, etiologies, clinical manifestations, and treatment recommendations for these two broad classes of osmoregulatory disorders.