I actually have the OPPOSITE disorder myself, Hypothyroidism. So, I figure that the things that I'm supposed to avoid that decrease thyroid function might be something to include for your cat, in order to lower the thyroid production.
I don't know how many of these can be safely given to cats, but I'm giving you my complete list of thyroid decreasing foods/substances. You'll have to cross reference with the "do not feed" list and with a vet.
The following RAW foods contain Goitrogens, which decrease thyroid hormone production.
Cassava, Spinach, Canola, Turnips,
soybeans, Bamboo shoots, collards, Peaches,
pine nuts, Sweet potato, Kohlrabi, Rutabagas,
peanuts, Bok choy, kale, Cauliflower,
strawberry, broccoli, rapeseed, Caffeine,
millet, brussel sprouts, mustard greens,
pears, cabbage, Radishes,
These foods INCREASE thyroid function
Avocado
Saturated fat
These medications decrease thyroid function or hormone in the body.
Sulfadimethoxine
propylthiouracil
potassium perchlorate
iopanoic acid
Amiodarone
lithium
Phenobarbitone
phenytoin
carbamazepine
rifampin
I hope this helps. Don't forget to cross-check with a vet.