97% of all feline hyperthyroidism is caused by a non-cancerous tumor in the thyroid gland. Carcinoma of the thyroid gland causes the other 3%. The result of either kind is an increase in the production of thyroid hormone.

Clinical signs of hyperthyroidism in cats to look for are appetite changes, weight loss, increased activity, acting like a kitten again, increased drinking and urination, vomiting, diarrhea, heart problems, increased respiration rate, hair, coat, skin, and nail abnormalities, nighttime yowling, restlessness, confusion and behavioral changes, and high blood pressure.

So, what can make a tumor start growing in the thyroid gland? So far, the list of possible causes includes iodine levels in cat food, soy levels in cat food, a fire-retardant chemical found in some fish used in cat food, and Bisphenol A – an endocrine disruptor found in the coating of some pet food cans.