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Low Plasma Bicarbonate Level in Hyponatremia Related To Adrenocorticotropin Deficiency

Brookdale University Hospital and Medical Center, Brookdale, New York 11212

Address correspondence to: Tooraj Zahedi, M.D., F.A.C.E., Clinical Associate Professor of Medicine, State University of New York in Brooklyn, One Brookdale Plaza, SSI/Room 101A, Brooklyn, New York 11212-3198.

To the editor:

In the very interesting article by DeCaux et al. (1), a low serum bicarbonate is found to be a simple laboratory finding that can distinguish adrenocortical-induced hyponatremia from low sodium due to syndrome of inappropriate antidiuretic hormone. The low serum bicarbonate is postulated to be due to respiratory alkalosis. I disagree with this conclusion.

Mineralocorticoids and, to some degree, glucocorticoids increase the exchange of sodium for potassium and hydrogen ion in the distal tubules of the kidneys. Adrenocortical deficiency results in the accumulation of hydrogen ion in the body, which is in turn neutralized by bicarbonate molecules. Therefore, the low serum bicarbonate in these patients is caused by consumption of bicarbonate molecules by excess hydrogen ions and not by increase in exhalation of carbonic acid (CO₂) from the lungs.

This situation causes very mild acidosis that is easily corrected by the body’s other buffering systems and the serum pH remains normal. The normal serum pH of 7.42 is a testimony to this notion. In true respiratory alkalosis of the magnitude to reduce the PCO₂ to 30, as was the case in the patients in this article, the pH is much higher, even with compensation.

Received February 26, 2004.

References

1. DeCaux G, Musch W, Penninckx R, Soupart A 2003 Low plasma bicarbonate level in hyponatremia related to adrenocorticotropin deficiency. J Clin Endocrinol Metab 88:5255–5257[Abstract/Free Full Text]

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