Thirst a *later* sign of dehdyration?  What are the early signs?  I
cannot imagine thirst is a later sign.  Maybe in the desert, but I
cannot believe that the human body's response would be so poorly
developed.  Thirst results from brain receptors responding to increased
osmolality of the plasma, and is very sensitive to slight increases in
plasma osmolality.  Another is decreased urine output, though this is
not a signal the body pays attention to.  It just does not seem
evolutionarily productive for headache to be the first sign of
dehydration.

Jack Newman, MD, FRCPC