Thinking of your Ava and her diagnosis. Our Spuds (16 yo feline) started losing weight last year, had what I considered fur loss/thinning fur (plus he was pulling it out), was cranky, and was so hungry all the time. He was diagnosed with hypothyroidism and was put on Methimazole. Within a very short time, things started turning around for him for the positive. He's now been on the medication for about 6 months and is a totally different kitty. His fur is back to what it was and his mood & hunger cravings are back to what was normal for him, and best of all is that his thyroid number is back to where it should be.
Will be thinking of your Ava and hoping that her diagnosis will be the very treatable thyroid issue.