Involuntary weight loss can happen even with an increased appetite or thirst. Some causes of this situation can include intestinal disorders that cause lack of absorption of food (like chronic diarrhea), endocrine disorders that cause the body to burn more energy (like hyperthyroidism), and uncontrolled diabetes, which causes the body to lose excess calories by spilling sugar into the urine.
Diabetes is a disorder of elevated blood sugars. Sometimes sugars are just mildly elevated at diagnosis. But sometimes blood sugar can become quite elevated before the diagnosis is made.
As the blood sugar level goes up, the body cannot reabsorb all of the sugar that is naturally filtered through the kidneys, and the sugar is spilled in the urine. This causes people who have very high sugars to be very thirsty, and to have to urinate very frequently. Patients will often have sudden significant weight loss associated with these symptoms. These same patients will be so thirsty they will often drink sugary drinks (such as sodas, juices or sweetened coffee drinks) which causes the sugars to be even higher, and the weight loss to be more severe. It can become a sudden dangerous cycle.
If you suddenly develop significant involuntary weight loss, especially associated with significant thirst or an increased need to urinate, it is critical to see a health care provider as soon as possible.