The thyroid gland puts out a hormone called thyroxine, which helps the cat's body digest food and burn through the energy that food produces. When the thyroid doesn't put out enough thyroxine, this is called hypothyroidism. This is rare in cats. But hyperthyroidism--where the thyroid puts out too much thyroxine--is far more common, especially in older cats.
In hypothyroidism, the cat will lose his or her appetite. But in hyperthyroidism, the cat will seem constantly hungry and will be prone to vomiting up food that was eaten too quickly.
In hypothyroidism, the cat may gain weight, even though the appetite decreases. But in hyperthyroidism, the cat will quickly lose weight.
In both hypothyroidism and hyperthyroidism, the cat's fur will thin and may feel harsh. Hyperthyroid cats may also develop bald patches.
Bowel Movement Changes
In hypothyroidism, the cat will become constipated and have smaller bowel movements. But in hyperthyroidsim, the cat usually gets foul-smelling diarrhoea.
Hypothyroid cats become more sluggish, but hyperthyroid cats often become more active in a search for more food, sometimes pacing about the home all night.