Dehydration is total body fluid loss. This can come about from intense exercise and sweating or commonly arises from intractable nausea and vomiting or diarrhea. If the dehydration is from exercise, first signs and symptoms could include copious amounts of sweat, intense thirst, intense body heat. Later signs and symptoms could be fatigue, muscle cramps and disorientation. Heat stroke can also occur if the person is exercising or if they are outside in the hot sun.
If the dehydration is caused by repeated episodes of vomiting or diarrhea, the first signs and symptoms could be intense fatigue and muscle weakness. Hypotension can also be present.
When someone is dehydrated, an area of concern is the loss of electrolytes such as sodium, potassium, chloride, calcium, and others. Loss of these ions is what causes the fatigue, muscle weakness, and disorientation.
The first sign is the most ignored sign: thirst. As soon as you notice you feel thirsty, you should consume some liquids with electrolytes because you are dehydrated.
The second is the most misunderstood sign: a headache. Headache's are very misunderstood things by most people because the range of possible reasons for having a headache is quite expansive. Tension and dehydration headaches are by far the most common.
Other signs of dehydration (in otherwise healthy adults):
Dark urine or the inability to urine
Dehydration if left untreated can lead to shock.