Thirst doesn’t automatically mean dehydration.
One of the most bogus myths out there is the idea that once you start to feel thirsty, you’re already dehydrated. The truth is that what we know as “thirst” is a complex physiological process that works to prevent dehydration. Your brain actually has a number of very precise mechanisms for monitoring water in your cells, with each of these being sensitive enough to detect when the water in your body decreases (or increases) by a few hundred millilitres.