Persistent SIADH-like Hyponatremia Requiring Tolvaptan Despite Glucocorticoid Replacement in Rathke's Cleft Cyst-Associated Hypophysitis: A Therapeutic Dilemma.

A 52-year-old man presented with severe hyponatremia and Rathke's cleft cyst (RCC)-associated hypophysitis with enlargement of the pituitary stalk. Hyponatremia persisted despite treatment with high-dose hydrocortisone, hypertonic saline, and loop diuretics, ultimately requiring low-dose tolvaptan. Hyponatremia associated with RCC is usually attributed to secondary adrenal insufficiency (SAI), in which glucocorticoid replacement generally restores free water excretion but may unmask arginine vasopressin deficiency (AVP-D). In the present case, however, persistent AVP-mediated water retention beyond SAI appeared to predominate, suggesting AVP dysregulation associated with inflammatory pituitary involvement. This case highlights the therapeutic dilemma of balancing masked AVP-D against persistent AVP excess in hyponatremia associated with RCC.