This is a significant weight loss and requires more evaluation than just routine bloodwork. I would certainly be looking for cancer, and also to see if the melanoma had metastasized. This would require x-rays and other studies. You could also have some kind of problem with your intestines that is interfering with proper digestion of food (even a tapeworm!), some chronic infection such as tuberculosis, a collagen vascular disease, hyperthyroidism, and a variety of other conditions. Depending on other symptoms, your doctor may be able to determine what the best tests to start with are. Certainly a chest x-ray should be done. You may need specialized tests to see if lymph nodes are enlarged. This degree of spontaneous weight deserves an extensive work up. Hope this helps.