I have often made the experience that when I’m thirsty, drinking pure water will not help it, but somehow make it worse. Drinking juice or literally anything else though (except coffee, alcohol, …) does improve it.
In my own personal experience, this insatiable thirst is due to low blood sugar (or high? getting those confused) and if I don’t get some kind of sugar I get a headache. But it is not the same as regular thirst.
And it sucks. I haven’t really pieced together what triggers it for me.