Pituitary Stalk Enlargement With Panhypopituitarism and Arginine Vasopressin Deficiency in Lung Adenocarcinoma: Metastatic Infundibular Disease or Paraneoplastic Hypophysitis?

Paraneoplastic endocrine syndromes are well-recognised complications of lung malignancies, specifically small-cell lung cancer. The majority of patients with panhypopituitarism associated with lung cancer result from pituitary metastasis. In those with an enlarged pituitary stalk without obvious pituitary metastasis presenting with a combination of panhypopituitarism and arginine vasopressin deficiency (AVP-D), metastatic pituitary stalk infiltration should be considered rather than a paraneoplastic immune-mediated hypophysitis. We report a case of a 67-year-old gentleman with newly diagnosed lung adenocarcinoma, found to have panhypopituitarism before starting systemic anti-cancer treatment. Further hormonal tests revealed central hypothyroidism, hypogonadotropic hypogonadism and growth hormone deficiency. MRI of the pituitary demonstrated heterogeneous contrast enhancement without an obvious focal lesion. The pituitary stalk was slightly enlarged at the level of the optic chiasm, raising the possibility of hypophysitis. The patient was initiated on hydrocortisone and levothyroxine replacement therapy. Following hormone replacement, previously occult AVP-D became clinically apparent, manifesting as worsening dehydration, polyuria and polydipsia. The overnight water deprivation test confirmed AVP-D. The patient showed subsequent clinical improvement with desmopressin. Serum prolactin was mildly elevated, likely reflecting both pituitary stalk infiltration and concomitant metoclopramide therapy. This case discusses an exceptionally rare presentation of lung adenocarcinoma with isolated pituitary stalk infiltration, panhypopituitarism, and AVP-D before exposure to immune checkpoint inhibitors. Clinicians should be aware of this unusual manifestation, as prompt hormone replacement can significantly reduce morbidity and improve patient outcomes.