A 55-year-old woman who has a history of severe depression and had a radical mastectomy for carcinoma of breast one year back develops polyuria nocturia and excessive thirst - Laboratory values are as follows: Serum sodium 149 m Eq/L, Serum potassium 3.6 m Eq/L, Serum calcium 9.5 mg/dl, Glucose 110 mg.dl, Bun 30 mg/dl, Urine osmolarity 150 in Osm kg. The most likely clinical diagnosis would be -