Elderly cat bad reaction to thyroid med problem-experience,please?

last year,we adopted a 13 yr old cat ,we asked the shelter people for one who prolly would not be adopted w/out us.we have had a gr8 yr.suddenly,last month,she became thin and frail.but she ate well and was lively.we took her to the vet.she was hyperthyroid,not just a little but severely.she had a terrible reaction to her meds.she has an a[ppointment to see the vet 2morro.i am syringe feeding pedialyte and cat food.now,i feel so guilty i cant stand it.she was clearly ill,but enjoying life-eating and playing.a few days of meds and she stopped eating,moving.she looks like she is dying.i feel maybe i should have left best alone.like the medicine actually pushed her to the brink of death,not the illness.did i do wrong?should i have waited to see the vet?i need moral support.i feel like my insistance upon medical intervention is killing,rathewr than saving her.i am so guilty and sad and miserable.can any1 say anything to help me?has ur cat had bad reaction to med and survived?should i have waited?help some1.....i always thought going to the vet was a good thing,but i think the meds have almost destroyed her. ps-she is about 14 now.