Thirsty describes the urge to drink, while dehydrated describes an actual shortage of water in the body. You can feel thirsty long before you are dehydrated, and by the time you are properly dehydrated, thirst is often only one of several symptoms.
The core difference
Thirsty is a want or feeling; dehydrated is a physical state that thirst is warning you about.
- thirsty — wanting a drink: Running in the heat always makes me thirsty.
- dehydrated — actually short of water in the body: He felt dizzy and dehydrated after the long run.
How to tell them apart
Think of thirst as the alarm and dehydration as the fire it is warning you about. You feel thirsty first, as a signal to drink; if you ignore that signal for long enough, or lose fluid faster than you can replace it, you become dehydrated, with symptoms like dizziness that go well beyond simply wanting a drink.
The mix-up usually happens because the two feel connected: reach for water when you are thirsty, and you will rarely become properly dehydrated. But someone can be dehydrated without feeling especially thirsty, particularly older adults, whose thirst signal weakens with age, so the words are not interchangeable even though one often leads to the other.
