Dehydration is caused by water loss or by inadequate water intake. The commonest cause is failure to drink enough water, although severe perspiration is a significant cause in hot climates or bouts of hot weather. Fluid loss caused by episodes of diarrhea (especially in children and old people) can also cause rapid dehydration.
In addition, loss of sodium (salt) from the body typically leads to a loss of water. Therefore, abnormally low levels of salt in body fluids may cause a person to become dehydrated. Kidney (renal) failure and Addison’s disease (adrenal cortical insufficiency) are two conditions that may – if untreated – lead to sodium depletion and dehydration.