Polyuria is commonly attributed to diabetes mellitus, but arginine vasopressin deficiency (AVP-D), previously known as diabetes insipidus represents a distinct pathophysiology of vasopressin deficiency. When associated with pituitary stalk thickening (PST), the differential expands to include inflammatory, infiltrative, autoimmune, and neoplastic causes ^[1]. Accurate diagnosis is essential for guiding management in young adults, balancing surveillance versus invasive investigation. We present a case of spontaneous partial AVP-D in which polyuria was the presenting symptom and isolated PST was the only positive pathologic finding.

A 21-year-old male presented in 2024 with polyuria and polydipsia. He initially reported nocturia (1–3 episodes/night), late evening thirst, and anxiety about anhidrosis during exercise. No headaches, visual field deficits, or other neurological symptoms were reported. He had background Hashimoto’s hypothyroidism on thyroxine. Examination demonstrated euvolaemia and stable blood pressure. He was subsequently diagnosed with partial AVP-D.

Serum glucose was normal, excluding osmotic diuresis (HbA1c not performed). Biochemistry showed mild hypernatraemia (Na 146 mmol/L, serum osmolality 301 mOsm/kg) with inappropriately dilute urine (urine osmolality 181 mOsm/kg). Copeptin was low–normal (2.6 pmol/L) following water deprivation overnight. The remainder of his pituitary profile was unremarkable. MRI revealed mild pituitary stalk thickening and absent posterior pituitary bright spot; no microadenoma was identified. PET scan, CT chest/abdomen/pelvis, CSF analysis, tumor markers, CSF ACE, LDH, protein, glucose, and other autoimmune studies such as ANA, ANCA were unremarkable. MOG and AQP4 antibodies were negative. Serum IgG4 was mildly elevated and following multidisciplinary review, it was deemed not significant as there was no evidence of IgG4 disease and surveillance was recommended.

This case illustrates that polyuria in young adults is not always due to diabetes mellitus. Isolated pituitary stalk thickening presenting with partial AVP-D remains diagnostically challenging. Comprehensive evaluation excluded systemic, neoplastic, and autoimmune causes, supporting conservative management with biochemical monitoring and longitudinal surveillance.