Endocrine Abstracts

Arginine vasopressin deficiency (formerly known as central diabetes insipidus, or CDI) is an endocrine disorder characterized by insufficient secretion of arginine vasopressin (AVP), also known as antidiuretic hormone (ADH). This deficiency leads to impaired renal water reabsorption, resulting in significant polyuria and polydipsia. Copeptin, the C-terminal segment of the AVP precursor, has emerged as a promising biomarker for diagnosing AVP deficiency. We present the case of a 41-year-old male who presented with severe polyuria, polydipsia, and hypernatremia. Due to the strong suspicion of AVP deficiency, severe symptoms, and copeptin test taking 3 weeks to be processed, desmopressin treatment was started empirically while awaiting copeptin results, leading to significant improvement in symptoms and sodium levels. The copeptin level eventually returned at 22.6 pmol/l, above the diagnostic cut-off (≤4.9 pmol/l) for AVP deficiency. Further testing during hypertonic saline infusion test again showed elevated copeptin levels of 19.8, 21.3, and 22.6 pmol/l. Although copeptin results were pointing towards nephrogenic diabetes insipidus, there was a strong clinical suspicion of AVP deficiency, hence a water deprivation test was conducted. The initial urine osmolality was 78 mOsm/kg despite a serum osmolality of 320 mOsm/kg. After desmopressin administration (2 mg IM), urine osmolality increased to 884 mOsm/kg, confirming AVP deficiency. Retrospective copeptin measurement during the water deprivation test showed a level of 12.2 pmol/l. We plan to monitor the copeptin levels every three months to see if the levels eventually drop below 4.9 pmol/l. The laboratory confirmed that copeptin results were accurate with no assay interference. This case highlights the limitations of copeptin testing, particularly in early cases or partial AVP deficiency. Despite high Copeptin test diagnostic accuracy of 97%, confirmatory testing such as water deprivation test should be considered when clinical judgment doesn’t align with the test results.