Classic signs of diabetes mellitus show up when any important function is lost. Blood glucose levels may become elevated (a condition called hyperglycemia) while cells starve because insulin is not available to move the sugar inside the cells where it is needed. The hungry cells, especially brain cells, signal the body to eat more, resulting in an excessive appetite (polyphagia).
In spite of the increased food intake, the cat loses weight because the cells can't benefit from the excess food without insulin. Urine production increases (polyuria) as the body attempts to get rid of excess sugar through the kidneys; and as sugar is eliminated, water goes with it. The cat drinks excessively (polydipsia) to compensate for the lost water. Increased sugar concentration in the bloodstream stimulates thirst.
The cat's coat may become unkempt and scaly. Interactions with owners may change. Urinary "accidents" may happen due to increased urine production and urinary tract infections, which are commonly seen in diabetic cats.