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Thank you for using Just Answer. My name is Rebecca; I am a veterinarian. Please give me a moment to type my answer.
I am sorry to hear Jake is hyperthyroid, and now is acting aggressive and hissing at the other cats.I see he was diagnosed last year, and is on the Y/D diet (is that the low iodine diet he is getting?). Has he had a blood test since last year, to check to see if his hyperthyroidism is controlled?If his thyroid is still high, he could have high blood pressure which can cause behavior changes in cats. He may be painful; cats in pain want to be left alone and will suddenly be unfriendly to cats they normally get along with. Having his blood pressure checked, his thyroid level measured, and a thorough physical exam would be what I would do if he were mine if these have not been done recently.Is Jake eating? Any vomiting? Has he lost or gained weight since starting on the low iodine diet?
I have not heard back from you; I hope you were able to see my responses.Please let me know if my answer was helpful and how Jake is doing. If you have more questions please let me know.