This is a retrospective cohort study to assess the safety and efficacy of low first dose of Tolvaptan and low or standard second dose of Tolvaptan in patients with moderate to severe hyponatraemia associated with SIADH not responding to conservative means of hyponatraemia management.

Patients are treated as part of standard clinical care. There is growing evidence that treating patients with SAIDH induced hyponatraemia using a low dose of Tolvaptan with 7.5mg (below licensed lowest 15mg daily dose). This is the largest study to date and seeks to validate the efficacy and safety or this lower than approved dose of Tolvaptan in patietns who only need a first dose but also in patients who need a second low or srandard dose of Tolvaptan.

Inclusion Criteria:

• All patients presenting to Accident & Emergency department or seen as inpatients with moderate to severe hyponatraemia (sNa: <125mmol) associated with SIADH and not responding to conservative measures of hyponatraemia management.
• Patients of all genders aged 18-90 y.o

Exclusion Criteria:

• Pregnant women and patients aged < 18y.o
• Hyponatraemia not associated with SIADH

Patients admitted to hospital through Accident & Emergency department or those who developed inpatient moderate to severe refractory hyponatraemia associated with SIADH and needed treatment with Tolvaptan.