Furthermore, despite SIADH being challenging to investigate due to its heterogeneity, a ratio of copeptin to urinary sodium below 30 pmol/mmol helps verify euvolemic hyponatremia; copeptin is the C-terminal fragment of the ADH hormone produced in our body, and it is thus considered a reliable surrogate of activated ADH, at the same time reflecting the intravascular volume state [91]. Here, AVP is linked to inappropriate ADH syndrome.