AVP and inappropriate ADH syndrome: However, HN in encephalitis may also be caused by SIADH, with an incidence of 5% among TBE patients (Czupryna et al., 2014); their further research showed that urinary sodium excretion to plasma copeptin ratio may be used as a potential biomarker of SIADH in patients with TBE (Czupryna et al., 2017).