Oncogenic osteomalacia and inappropriate antidiuretic hormone secretion due to oat-cell carcinoma

The syndrome of tumor-induced osteomalacia has been previously thought to occur only in association with mesenchymal tumors, although one report has linked prostatic carcinoma with the syndrome. We report the case of a patient who presented first with the clinical and biochemical features of the syndrome of inappropriate antidiuretic hormone secretion, and then oncogenic osteomalacia. The first syndrome was characterized by headaches, nausea, and vomiting; serum sodium determinations ranged between 107 and 118 meq/L with simultaneous urine spot sodium concentrations of 100 to 116 meq/L. The circulating antidiuretic hormone level was markedly elevated to 261.5 microU/mL. The osteomalacia was discovered incidentally when depressed serum phosphorus levels of 1.2 to 1.7 mg/dL were noted in association with 24-hour urine phosphorus excretion exceeding 1000 mg/24 h. Undecalcified tetracycline-labeled bone biopsy samples confirmed oncogenic osteomalacia. Only afterward was a small-cell carcinoma of the lung identified as the likely source of both of these syndromes.