There are medications that can help if she has hyperthyroidism. Sometime hyperthyroidism can cause heart murmurs that can resolve when the thyroid problem is treated. Sometimes the heart murmurs are separate. If she is hyperthyroid it is very important to treat her. There are several options: medication, surgery, or radioiodine (I-131). I would suggest starting with the medication. Be sure that the vet starts her on a low dose. If it isn't enough (determined through another blood test), you can increase it. Starting low will reduce the risk of side effects. If you can afford it, you may want to consider the radioiodine treatment. It is a lot of money upfront, but it can cure the hyperthyroidism. Unfortunately, your cat has to go to a facility to receive the treatment, and they have to stay there until their radiation levels drop afterwards, and I have heard that England's facilities keep the cats for several weeks. It's something you may want to do more research about.