A case of syndrome of inappropriate secretion of antidiuretic hormone (SIADH) with low plasma concentrations of antidiuretic hormone.

A 66-year-old Japanese man presented with persistent hyponatremia without polydipsia and polyuria. Laboratory examination showed serum sodium of 117 mEq/l, plasma osmolality 239 mosm/kg, urine sodium 108 mEq/l, urine osmolality 577 mosm/kg, and normal levels (less than 2.0 pg/ml) of serum antidiuretic hormone (ADH). ADH release was regulated normally with changes in plasma osmolality. No obvious cause for the syndrome of inappropriate secretion of ADH (SIADH) could be detected. However, 20 months later, the patient had bouts of hematuria and was found to have cancer of the urinary bladder. Increased renal sensitivity to ADH was suspected as the underlying mechanism of SIADH.