Comparisons between the group of patients with AVP deficiency and those with SIAD showed statistically significant differences for Day 1 copeptin levels (median = 2.5 pmol/L and IQR = 2.0–3.0 pmol/L versus median = 3.8 pmol/L and IQR = 2.5–5.8 pmol/L, p = .04), Day 8 copeptin levels (median = 2.0 pmol/L and IQR = 1.6–2.2 pmol/L versus median = 3.7 pmol/L and IQR = 2.4–5.9 pmol/L, p < .01) but not for Day 2 copeptin levels (median = 2.3 pmol/L and IQR = 2.0–2.6 pmol/L versus median = 3.3 pmol/L and IQR = 1.6–3.9 pmol/L, p = .22). This evidence concerns the gene AVP and neurohypophyseal diabetes insipidus.