Hypercalciuria in the syndrome of inappropriate secretion of antidiuretic hormone.

Mild hypocalcemia was observed in 6 out of 12 patients with SIADH associated hyponatremia, this was in fact related to low albumin levels resulting partly from body fluid dilution. In the 7 SIADH patients where it was measured, we observed an increased fractional calcium excretion (3.2 +/- 1.7%) as long as the patients were hyponatremic. This was corrected by water restriction (0.73 +/- 0.4%, p less than 0.01). We suggest that volume expansion was responsible for the increased calcium clearance, and not hyponatremia by itself, since in volume depletion hyponatremia, calcium clearance was within the normal range. Mild hypocalcemia and hypercalciuria is a common finding in SIADH-associated hyponatremia.