Q. Was violently sick,then mewing and going into spasm. These continued for about 2 hours,getting milder each time. She is 16 and on100mg Felimazole day
DAVID ELBEZE, DVM, MRCVS
A. Sickness could be a side effect of the Felimazole, especially if it was started recently. the other possibility is that the hyperthyroidism isn't controlled properly at the moment and she might need a tweak in the dosage.
I recommend you take her to the vet for a blood test (thyroid + kidneys), this will probably give you the answer and the vet will adjust the treatment.