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Hyperthyroidism is caused by a benign tumor on the thyroid gland which produces an overactive metabolism (excess thyroid hormone). This starts as making the cat very hungry yet losing weight, as they are burning off much more than they can ever take in. Eventually it becomes worse, and the cat literally starts wasting away, malnourished. This also causes quite a strain on the other organs of the body which are trying to compensate for it. Most often, the heart is affected first, having trouble trying to keep blood pressure stable and when the heart becomes overworked, eventually heart failure ensues. This can lead to sudden death or things like fluid build up in the body and poor circulation to limbs (sometimes even paralysis). Often the kidney is affected as well, sending the animal into kidney failure due to the changes in the blood pressure (and the kidney requires the highest blood pressure of any organ in the body). This causes toxicity to the brain eventually from the building up of toxins which should have been excreted through the bladder. There's no telling what one particular cat would suffer first, but weakness and a sick, nauseated feeling is more likely than any pain. Weakness eventually becomes severe.
Some cats with hyperthyroid only have it mild and if already quite old, other age related changes in the body may become the problem well before the hyperthyroid problem becomes bad. Every cat is individual, and treating is determined by owners based on what they choose for their cat and what fits with their life.
Hope this helps. Let me know if I can help further,