We are going through this with our cat Simba, if we took him to the vet at age 22, 23, maybe older we don't know exactly how old but he would have a heart attack, so we ordered Hyperthyroid supplement from Best Life 4 Pets, a holistic supplement. There also is also Thyroid Support Gold by Pet Wellbeing, they both get good reviews and you don't need a vet or a prescription for them, just Google them and you will see them.
Best of luck with your cat!!
Best of luck with your cat!!