Hi. I'm Dr. Lee and will do my best to help.
With a young cat and losing weight yet eating a lot, hyperthyroidism is possible, but would be a little young for that. I would be concerned about intestinal absorption problem. I would suggest to ask your veterinarian to run a specific blood testing for cobalamin and folate (usually requires sending to the lab in Texas to run). As well, I would suggest to ask your vet to start her on vitamin B12 injection. Some cats can run into vitamin deficiency and just need the supplement. As well, if running the test and using vitamin B12 and there is no improvement, then you need to consider seeing an internal medicine specialist for consultation. You can find one here: www.acvim.org
Please let me know if you have any questions or concerns. Thanks.