Glandulars - although thyroxine (T4) is removed from thyroid glandular products, please call the company first to make absolutely sure. One product that has been used a lot with my list cats is Thytrophin PMG by Standard Process. Depending on what other issues your cat has, other glandulars might also be helpful e.g. cardiac or renal glandulars.
CoQ10 30mg minimum, preferably 50mg daily - CoQ10 levels are lowest in humans with heart and also hyperthyroid issues. It also has been found to help the kidneys (see reference in CRF article), something with which many cats with hyperthyrodism could use help.
L-Carnitine - 250-500mg for both heart and thyroid; this one build up slowly because it can give some kitties the runs. It also has a citrus taste many cats dislike, yet others don't seem to notice it, so cat mileage varies. References to 2 papers below.