Dehydration is a generally dangerous condition for any animal, in which the tissues are low on water. It is particularly likely in poorly-regulated or hyperglycemic diabetics, and also particularly dangerous for them, because it can quickly trigger diabetic ketoacidosis.
The loss of fluids from the body is divided into two major categories--sensible and insensible. Sensible means able to be measured in some way; urination, defecation and vomiting are all in the sensible category because of their ability to be measured. Breathing is classed as insensible because while there is some fluid loss to the system from it, it's not possible to measure accurately.  In the case of fever, however, it's possible to say that there are insensible fluid losses that increase by 7% for each degree of higher than normal temperature. 
Dehydration can change the way subcutaneous insulin is absorbed. , causing either hyperglycemia or hypoglycemia  It can also create false positive or false negative urine ketone test results. 
Excessive thirst (medical term polydipsia [pah-lee-DIP-see-uh]; abreviated as PD) is a symptom of diabetes. Diabetic animals often drink incessantly because they are dehydrated from the cell-dehydrating effects of hyperglycemia, plus the effects of their bodies casting off the excess glucose through urination, taking hydration with it. The process also removes electrolytes  the body needs to function properly such as potassium,  sodium and chloride  also.
When the skin at the back is lifted, a dehydrated animal's doesn't fall back into place quickly. Serious dehydration (loss of 10-12% of body fluids) means the pulled up skin just stays there and doesn't go back into place. At this point, the animal may go into shock; dehydration of 12% or more is an immediate medical emergency. 
In any case of dehydration, check frequently for ketones. Mild dehydration may be possible to remedy with lots of water; if this isn't working, the next step is subcutaneous fluid injections, usually performed by your vet.
|Clinical Signs of Dehydration |
|Based on percentage of body weight,|
not percentage of fluid loss
|< 5% (mild)|
|slight loss of skin elasticity|
|definite loss of skin elasticity|
|slight prolongation of capillary refill|
|slight sinking of eyes into orbit|
|slight dryness of oral mucous membranes|
|tented skin stands in place|
|prolonged capillary refill|
|eyes sunken in orbits|
|dry mucous membranes|
|possible signs of shock|
|12-15% signs of hypovolemic shock, death-emergency |
Untreated dehydration can cause the blood to be more hypertonic, which in turn can suck water from the cells causing more dehydration. Hypovolemic shock is a life-threatening medical condition in which the heart is unable to pump sufficient blood to the body, due to loss of fluids through either dehydration or bleeding. 
- ↑ 1.0 1.1 Assessing Dehydration Status. Washington State University.
- ↑ 2.0 2.1 Fluid Facts. Washington State University College of Veterinary Medicine.
- ↑ Hyperglycemia. FAQS.org.
- ↑ Pediatric Endocrine Emergency Answer Sheet. American College of Emergency Physicians.
- ↑ Diabetes and Travel-page 4. DiabetesNow-UK.
- ↑ Ketosis and Dehydration. North American Veterinary Conference (2003).
- ↑ Stoeppler, Melissa Conrad. What Are Electrolytes?. MedicineNet.
- ↑ Potassium Requirements & Deficiencies. Pet Education.
- ↑ Sodium & Chloride Requirements & Deficiencies. Pet Education.
- ↑ Wortinger, Ann (February 2001). Electrolytes, Fluids and the Acid-Base Balance. Veterinary Technician.
- ↑ Water: A Nutritional Requirement. Pet Education.
- ↑ Assessing Dehydration Through Skin Elasticity in Dogs and Cats. Pet Education.
- ↑ Dehydration. Washington State University School of Veterinary Medicine.
- ↑ 14.0 14.1 Hypovolemic shock. University of Maryland Medical Center.
- Assessing Dehydration Through Skin Elasticity in Dogs and Cats Pet Education
- Water: A Nutritional Requirement Pet Education.com-Drs. Foster & Smith
- Fluid Therapy in Small Mammals North American Veterinary Conference 2005