Immature ovarian teratoma with hyponatremia and low serum vasopressin level

Hyponatremia is often caused by the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). Hypersecretion of vasopressin from malignant tumors can be considered a cause of SIADH. Most of these ectopic productions of vasopressin are complications of small cell lung cancer. Cases concomitant with ovarian tumors are very rare, and a specific causative substance from the ovary is often unknown. A 16-year-old woman was diagnosed with an ovarian tumor. She developed hyponatremia that was resistant to medical treatment, but immediately improved after surgical resection of the tumor. Her diagnosis was SIADH caused by an ovarian tumor; however, her serum vasopressin level was normal. It is possible that a vasopressin-like substance causing SIADH was secreted by either nervous system tissue within an immature teratoma or small cell lung cancer. We should be cautious when SIADH is a complication of an ovarian tumor.