Why does Hyperglycemia cause Hyponatremia?

The human body will maintain a serum [Na⁺] between 135 and 145 mEq/L. Hyponatremia in the context of hypergylcemia is a commonly discussed problem. It presents with hyponatremia (<135 mEq/L) in the setting of increase plasma osmolality. (Posm >295 mOsm/kg H2O)

HOW DOES IT WORK? Remember hyponatremia is always a problem with free water. Glucose acts as an osmole (won’t enter cell without insulin) and draws water from the intracellular fluid (ICF) into the extracellular fluid (ECF). Thereby effectively diluting the ECF [Na⁺].

CORRECTION FACTOR: plasma Na⁺ concentration falls by ~1.6 mmol (Katz, 1973) to 2.4 mmol (Hillier et al in 1999) for every 100 mg/dL (5.55 mmol/L) increase in glucose, due to glucose-induced water efflux from cells; this “translational” hyponatremia resolves after correction of hyperglycemia. CALCULATOR 

NOTE: This dilutional psuedohyponatremia can occur in other instants.

REFERENCES

1.  Katz MA. Hyperglycemia-induced hyponatremia–calculation of expected serum sodium depression. N Engl J Med. 1973 Oct 18;289(16):843-4.
2.  Hillier TA, Abbott RD, Barrett EJ. Hyponatremia: evaluating the correction factor for hyperglycemia. Am J Med. 1999 Apr;106(4):399-403.
3. Tintinalli’s Emergency Medicine: A Comprehensive Study Guide, 8e. Roberta Petrino; Roberta Marino.  Chapter 17: Fluids and Electrolytes
4. Rose RD: Clinical Physiology of Acid-Base and Electrolyte Disorders, 3rd ed. McGraw-Hill, 1989
5. Harrison’s Principles of Internal Medicine, 19e. David B. Mount. Ch 63: Fluid and Electrolyte Disturbances.