Determining the cause of hyponatremia in adults

IVIG: intravenous immune globulin; TURP: transurethral resection of the prostate; BP: blood pressure; ACTH: adrenocorticotropic hormone; ADH: antidiuretic hormone; TSH: thyroid-stimulating hormone; SIADH: syndrome of inappropriate antidiuretic hormone secretion.

* A simple and convenient correction of the serum sodium for hyperglycemia is as follows: Add 2 mEq/L to the serum sodium for every 100 mg/dL (5.55 mmol/L) of serum glucose above the normal value.

¶ Impaired water excretion in renal failure occurs if there is severe impairment in glomerular filtration rate. Patients with mild-to-moderate impairment in glomerular filtration rate are typically able to excrete water loads. The measured plasma osmolality may be high in patients with renal failure because of high urea concentrations. However, urea is an ineffective osmole, and such patients have hypotonic hyponatremia even if the plasma osmolality is normal.

Δ Thiazide-induced hyponatremia may be protracted. An extensive evaluation for other etiologies can be delayed for several weeks in mildly hyponatremic patients.

◊ Patients with hypovolemic hyponatremia due to diuretics may have a low urine sodium if the effect of the diuretic has worn off.

§ If the serum sodium is 125 mEq/L or less, we do not give isotonic saline. In such patients, the evaluation can be delayed until the sodium is slowly raised to higher levels.

¥ Although patients with hyponatremia due to heart failure or cirrhosis will usually have edema that is clinically apparent, hypovolemia may not always be apparent by clinical exam. Thus, in a patient who appears to be euvolemic but whose urine chemistries are consistent with hypovolemia, infusion of isotonic saline (eg, 1 liter over one hour) can be helpful.