Source: Chessman KH, Haney J. Disorders of sodium and water homeostasis. In: DiPiro JT, Talbert RL, Yee GC, Matzke GR, Wells BG, Posey LM, eds. Pharmacotherapy: A Pathophysiologic Approach. 10th ed. New York, NY: McGraw-Hill; 2017. http://accesspharmacy.mhmedical.com/content.aspx?bookid=1861§ionid=146061710. Accessed April 18, 2017.
Homeostasis: Physiologic processes that maintain a relatively stable equilibrium between interdependent elements.
Arginine vasopressin (AVP), commonly known as antidiuretic hormone, released from posterior pituitary when plasma osmolality increases by 1–2% or more.
Classified as isotonic, hypertonic, or hypotonic depending on serum osmolality (Figure 1).
Hypertonic hyponatremia associated with increased serum osmolality, most commonly due to hyperglycemia.
Hypotonic hyponatremia most common with many potential causes.
Hypovolemic hypotonic hyponatremia associated with loss of ECF volume and sodium, with loss of more sodium than water.
Euvolemic hypotonic hyponatremia associated with normal or slightly decreased ECF sodium content and increased total body water and ECF volume.
Hypervolemic hypotonic hyponatremia associated with increase in ECF volume in conditions with impaired renal sodium and water excretion, such as:
Diagnostic algorithm for the evaluation of hyponatremia. (HF, heart failure; EABV, effective arterial blood volume; SIADH, syndrome of inappropriate antidiuretic hormone; UNa, urine sodium concentration [values in mEq/L are numerically equivalent to mmol/L]; UOsm, urine osmolality [values in mOsm/kg are numerically equivalent to mmol/kg].) Reprinted with permission from Wells BG, DiPiro JT, Schwinghammer TL, et al. Pharmacotherapy Handbook. 10th ed. New York, NY: McGraw-Hill; 2017.