Occult pulmonary malignancy in syndrome of inappropriate ADH secretion with normal ADH levels

Although the syndrome of inappropriate ADH secretion (SIADH) has many causes, principally pulmonary, central nervous system or neoplastic disease, and drugs, patients may present with SIADH in whom the etiology is not readily evident. We measured serum ADH levels in such an individual in both the eunatremic and water-loaded states and found levels to be undetectable despite failure to dilute the urine. A small oat cell pulmonary carcinoma was ultimately diagnosed with lung tomograms and cytology. Following a partial response to water restriction, demeclocycline was effective in producing a water diuresis that restored the serum sodium concentration to normal. Patients with clinical SIADH but low serum ADH levels can harbor a malignant or benign process that, notwithstanding the low ADH levels, may still remain responsive to demeclocycline, suggesting either neoplastic production of a biologically-active, immunologically-inactive ADH-like peptide, or increased renal tubular sensitivity to ADH.