The loss of fluid and electrolytes through sweat can lead to dehydration in some circumstances.
When excessive sweating is caused by vigorous or prolonged physical activity (i.e. during endurance sport or exercise), dehydration may occur. This can happen if adequate fluid consumption (with an appropriate hydration fluid) does not occur.
In cases where excessive sweating is due to a fever, there is also a chance of dehydration if adequate hydration is not achieved.