I don't have a hard and fast answer except to ask, is kitty able to do things she likes to do anymore?
Miss Teia is down to 6#, was 12# in her younger years with us (we adopted her at age 11, she will be 19 in June), has been at the high end of normal on the T4 count for a few years, finally is in mild hyperthyroid territory. We'll be starting a medicine in gel form that we can rub on her ear once a day.
Despite this, Teia eats well, grooms normally, has good box habits, is social, moves okay...which are our baseline observations for how kitty is doing. Should the time come when she is no longer able to do these simple things, it will be our sad duty to offer her the Final Kindness and send her Over the Rainbow. I dread that day, have had to do it before, and it hurts every time, even though it is the right thing to do.
Hope this helps.
I don't know if animals are the same as humans, but I know when I start having symptoms I have to go back to the doctor for a blood test for my Thyroid. If the levels are off, they adjust the dosage of the medicine I take. Maybe that would work for your cat?