SIADH: Fluid out of balance
Ann Crawford PhD, RN
Helene Harris MSN, RN

September 2012 
Volume 42  Number 9
Pages 50 - 58
  PDF Version Available!

AFTER A TRAUMATIC brain injury from a fall at home 3 days ago, SL, 62, is admitted for observation. Entering his room to perform an assessment, his nurse finds him disoriented, apprehensive, tachycardic, and complaining of nausea. His I.V. and oral fluid intake for the past 24 hours was 3,500 mL. His urine output during that time was only 1,050 mL, and his serum sodium level is 120 mEq/L. His signs and symptoms suggest a fluid imbalance, probably related to a problem with antidiuretic hormone (ADH), also known as arginine vasopressin.Two disorders of water balance are related to abnormal levels of ADH: diabetes insipidus and the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Both conditions can be fatal if not promptly identified and appropriately treated. This article describes the underlying pathophysiology, clinical manifestations, and management of SIADH. Diabetes insipidus is beyond the scope of this article.The endocrine system regulates and integrates the body's metabolic activities to maintain internal homeostasis. Endocrine glands secrete hormones that are carried by the circulation to target cells, where they link with a target hormone receptor to produce a series of biochemical responses. Hormones may be stimulatory or inhibitory, and their effects may be widespread and general (as with insulin) or organ-specific (as with prolactin, which stimulates the mammary glands). Hormones are transported via the circulation because most endocrine glands are located in a different area of the body than the target tissue.1,2 (See Locating the endocrine glands.)Hormone production is regulated through a feedback system. Sensors in the hypothalamus monitor the various hormone levels and stimulate or suppress production according to the body's needs. Endocrine hormone secretion is generally stimulated through a negative feedback system that stimulates or stops hormone secretion as needed to reestablish homeostasis.1,2 (See How negative feedback regulates

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