Will you share your knowledge about dealing with hyperthyroid cats?

My 13 year old spayed female was recently diagnosed with hyperthyroidism. She seems to experience few symptoms – vomiting once to twice a week and diarrhea once a week. She is beginning to get a noticeable case of the skinnies, especially around the hips, but nothing too bad – her spine and ribs are not showing or anything like that. Her appetite has tapered down somewhat – she tends to pick at her food over a long period of time rather than scarf it down like she used to.

My question is, what can I expect if I choose to not treat this? It’s not whatsoever an issue of money; I can afford the pills and yearly bloodwork. I also don’t mind pilling her twice daily. It’s a matter of something that the best physician I know (my mom) has told me: Treat the patient, not the disease. Which is to say, if the symptoms are not hurting her, then the treatment may turn out to be harsher than the disease itself would be, just on it own. And the side effects of the hyperthyroid meds seem to be kind of nasty: vomiting, anorexia, fever, anemia, and lethargy – and from long-term use, there’s a small possiblity of liver damage and bone marrow suppression (so the internet tells me). And I know that at least in humans, hyperthyroidism is sometimes left untreated if the symptoms are not too bad.

I need more information in order to make this decision. Do symptoms worsen as time goes on? Will having the disease shorten her life, and does treatment have the potential to lengthen it?

That’s just the most important question – I welcome any and all suggestions and stories. Thank you, jellies!