It would be a good idea to bring a fecal sample to your veterinarian to check for parasites or poor absorption. If you can get a clean urine sample and have it checked, it could reveal whether there is diabetes, infection, or poor kidney function. Any of these conditions can contribute to weight loss and hair loss.
Please let us know what you find out and we can help you with some suggestions to help your cat feel better and improve the skin and coat. Generally Derma-IonX is used for skin problems, but if there are kidney complications I would use Dia-IonX. For general good health I would use probiotics and Omega 3,6,9.
Please let us know if you have any other questions.