Hyperthyroidism in older cats typically causes weight loss, restlessness and increased appetite. Some of these cats become really grumpy and difficult to deal with, so it not only wears them out, it can wear you out, too. In addition, if untreated, it often leads to a type of heart disease that certainly is life threatening, and not a nice way for her to end her days.
Treatment usually starts with tablets once or twice a day, otherwise there is a fairly straightforward operation to remove the over-active thyroid gland, or a few specialist centres offer course of radiotherapy. Interestingly and slightly controversially, recent research suggests that restricting the iodine intake in the diet can reverse hyperthyroidism
So the short story is that hyperthyroidism is no fun. If your cat could be affected talk to us about having a blood test.