Rathke's cleft cyst presenting with hyponatremia and transient central diabetes insipidus.

We describe an 18-year-old female who complained of general weakness, nausea, vomiting, headache, and lightheadedness. On physical examination, she was euvolemic without visual or neurological deficits. The striking biochemical abnormality was hyponatremia (125 mmol/l). This hyponatremia met the laboratory diagnostic criteria for the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). Two litres of normal saline were given per day for 4 days and this did not correct her hyponatremia. A spontaneous diuresis (6.6 l) developed in 1 day, causing a rise in her PNa of 26 mmol and a final PNa of 152 mmol/l. Magnetic resonance imaging revealed a dumbell-shaped intrasellar and suprasellar cyst. During transsphenoidal surgery, a Rathke's cleft cyst (RCC) lined with columnar epithelium containing mucoid material was resected. We speculate that the growing RCC may have produced critical compression over the stalk, thus contributing to the transition from SIADH with hyponatremia to transient central diabetes insipidus with hypernatremia.