The patient exhibits symptoms indicative of diabetes insipidus, characterized by polyuria and hypernatremia (Na+ levels at 155 mEq/L) with low urine osmolarity (200 mOsm/L). This condition is commonly seen post-hypophysectomy due to a deficiency in antidiuretic hormone (ADH) production, also known as vasopressin. In such cases, the treatment focuses on replacing the missing hormone to manage the high urine output and correct serum sodium levels.
The appropriate treatment is Desmopressin (DDAVP), a synthetic analogue of vasopressin, administered to provide continuous replacement of the absent ADH. This approach prevents dehydration and normalizes urine concentration and serum sodium levels.
Conclusion: Given the permanent nature of ADH deficiency post hypophysectomy, the patient requires DDAVP supplementation for lifelong to maintain homeostasis.