This happened to me at 18 and my cat when he was 16. Now he's turning 19 and I'm turning 43. He and I can confirm it is much better than dying. Mine was sliced and diced and I'm stuck taking a pill every day. For his they used a radioactive pellet to kill the extra tissue and he doesn't need to take anything.

This is happening much vastly more frequently with cats and some folks suspect a now banned chemical is to blame.

https://www.nytimes.com/2017/05/16/magazine/the-mystery-of-t...

If this happens to your cat there are 3 choices.

- Die soon

- Take Methimazole. It will partially control thyroid levels but not well so your cat still dies but later than option one.

- A vet implants a small radioactive pellet destroying the problematic tissue. Most cats will live as long as they otherwise would have and few have to take thyroid hormone.