Dehydration signs and symptoms can vary by age group, with adults and children showing very different symptoms. Often feeling thirsty means you are already dehydrated, which you want to avoid. Severe dehydration symptoms in young children and infants include dry tongue and mouth, no tears when crying, dry diaper for three hours or more, sunken eyes and cheeks, sunken soft spot on the top of the skull, and listless or irritable. In adults, symptoms of severe dehydration include extreme thirst, dark-colored urine, fatigue, dizziness, and confusion. Dehydration can also cause low blood pressure, increased heart rate, and reduced blood flow going out to your extremities. Doctors are typically able to diagnose severe dehydration based on symptoms alone, but they can also use blood tests and urinalysis to determine exactly how dehydrated you are and if there are other issues present as a result of dehydration.