dehydration, adrenal insufficiency, nephrosis, congestive cardiac-failure, liver disease.
The best way to know if someone is dehydrated is to look at the colour of their urine. It should be light in colour. The darker it is means it is more concentrated. There will be a dry mouth, not having enough urine to pee (less than 3 wet diapers for a baby would indicate the baby was dehydrated). If you pinch the skin and it takes a long time to spring back, that too can mean dehydration.