The most common clinical signs of hyperthyroidism in cats include weight loss, increased appetite (although some patients have decreased appetite), vomiting, increased thirst and urination, hyperactivity, and diarrhea. (aercmn.com)
Kittens with iodine deficiency show signs of hyperthyroidism in the early stages, with increased excitability, followed later by hypothyroidism and lethargy. (merckvetmanual.com)