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Background Traumatic brain injury (TBI) may lead to post-traumatic amnesia (PTA) and fluid balance abnormalities. This case describes an initial diagnosis of syndrome-of-inappropriate-antidiuretic hormone (SIADH), followed by Arginine-vasopressin (AVP) deficiency due to TBI. These conditions cause cognitive deficits, complicating Westmead PTA score interpretation and delaying rehabilitation.Case history A 25-year-old male sustained TBI and traumatic pneumonia in a motor vehicle accident, leading to confusion and amnesia. Initially, he had hyponatremia (120 mmol/L), low serum osmolality (265 mOsm/kg), elevated urine osmolality (360 mOsm/kg), and high urinary sodium (64 mmol/L) consistent with SIADH. Despite treatment, his Westmead PTA score remained persistently low. Subsequently, he developed hypernatremia (152 mmol/L), initially attributed to dehydration. However, persistent hypernatremia after adequate hydration, with polyuria, high serum osmolality (365 mOsm/kg) and low urine osmolality (78 mOsm/kg), raised suspicion of AVP deficiency, which was confirmed by a low Copeptin level of <2.6 pmol/L. MRI brain revealed hypothalamic post-traumatic changes. He was treated with desmopressin for AVP deficiency, leading to improvements in fluid balance, cognitive symptoms, and Westmead PTA scores.Conclusion This highlights the complex relationship between fluid balance disorders and cognitive recovery in TBI. Persistently low Westmead PTA scores should prompt evaluation for underlying metabolic or endocrine abnormalities.sathyajith.ambawatte@gmail.com