Medication suppresses the thyroid gland, if the correct amount of medication is given, the cat should not be losing weight and should not show other symptoms of hyperT (such as hyperactivity or ravenous appetite).
Cats do not usually die from hyperthyroidism if its treated promptly. The disease is completely reversible with medication, and unless the cat has other medical problems, she can likely lead a normal life for many more years.
When was the last time she took the cat to the vet to have a blood panel done? When a cat is first diagnosed, it usually requires several blood tests over several months and usually several adjustments to the amount of medication to get it right, and then the cat should have a blood test done about ever 6 months and/or whenever they start showing abnormal behavior to make sure her thyroid levels are where they should be and that all her other organs are functioning well. Medication should be split up so its give twice a day, every day, at the same time.
My cat, Trixie, has had hyperthyroidism for over 3 years. I've noticed that with her, vomiting is a sign that her medication needs to be adjusted.
I think the cat needs to see a vet and have a full blood panel including T4, then you will know more.