Fishtalk, Pugmama is right. I also had a cat, at ten years old, that lost weight so quickly over a three-week period it was startling. The fact it happened so quickly I almost didnt notice before it was too late and the fact he was a Norwegian Forest cat and very long fur. Although his fur had also changed dramatically too.
There is another treatment besides pills I opted for and it cures the cat of hyperthyroidism but you always have to watch kidney function closely. It is a radioactive iodine injection.
https://m.thyrocat.com/radioactive-iodine-therapy-for-cats.php
This was expensive but I felt the better option for Gabby because pills often cant regulate thyroid. This cures the problem. Plus you dont have to isolate the cat after returning home as sometimes is stated on various websites.
He recovered beautifully from this and lived to be 16 years old. Good luck. I love cats so much I cant find words. I understand your concern but do not wait to see a vet!
Thanks Blue for mentioning iodine therapy as well. Great outcomes with this treatment.
Thank you all for your concern over my cat. He is now up and about and ate a couple of shrimp.
I got this people. Do not send me any more email.
I will not address this matter again and sorry I brought it up.