Low plasma bicarbonate level in hyponatremia related to adrenocorticotropin deficiency

Patients with hyponatremia related to adrenocorticotropic deficiency are not easily distinguished by routine laboratory studies from patients with nonendocrine inappropriate secretion of antidiuretic hormone (SIADH). We wanted to investigate whether, in the routine biological analysis of such patients, some parameters could help to better identify this subgroup of hyponatremic patients. The biochemical profiles of 13 consecutive patients with hyponatremia related to ACTH deficiency were analyzed and compared with 30 consecutive patients with classical SIADH. Patients with adrenocorticotropic deficiency presented low uric acid and urea levels as in nonendocrine SIADH, but their total carbon dioxide was significantly lower (total CO(2), 20.5 +/- 3 vs. 25.5 +/- 2.4 mmol/liter; P < 0.001). Nine of the 13 patients presented a value lower than 22 mmol/liter, although this was not observed in the nonendocrine SIADH patients (P < 0.001). Arterial blood gas analysis was available in eight patients and showed a compensated respiratory alkalosis in most of them (pH 7.42 +/- 0.02; PCO(2), 30 +/- 5 mm Hg; HCO(3)(-), 20 +/- 2 mmol/liter; base excess, -3.4 +/- 1.8 mmol/liter). Aldosterone levels were much lower in ACTH deficiency patients during the hyponatremic state (33 +/- 40 pg/ml) when compared with the nonendocrine SIADH (120 +/- 60 pg/ml; P < 0.01). Correction of hyponatremia by cortisone therapy normalized total CO(2) and aldosterone levels. Low carbon dioxide level is a frequent observation in hyponatremia related to ACTH deficiency and could help to differentiate it from classical SIADH.