Idiopathic Arginine Vasopressin Deficiency With an Incidental Non-functional Pituitary Microadenoma in an Elderly Diabetic Woman.

We present a case of idiopathic arginine vasopressin deficiency (AVP-D) in an elderly woman with type 2 diabetes, who presented with polyuria, polydipsia, and nocturia. Laboratory findings confirmed a euglycemic state, hyperosmolar serum, and hypoosmotic urine. Magnetic resonance imaging of the pituitary revealed the absence of the posterior bright spot and an incidental small, non-functional pituitary microadenoma, without any compressive effects on the pituitary stalk. The diagnosis of AVP-D was confirmed through a water deprivation test and a positive response to desmopressin. Common secondary causes, including inflammatory, granulomatous, and structural pathologies, were excluded. The patient responded well to oral desmopressin therapy, resulting in normalization of serum sodium and urine osmolality. This case underscores the challenges in diagnosing the cause of polyuria in a diabetic patient and the challenges in determining the etiology of AVP-D.