Hello, Sarah and welcome.
I'm so sorry to hear that Shred is twitching and shaking and has you worried.
It's possible that he has developed a fever and this is causing the shaking. If you have a regular rectal or digital thermometer, you can take his temperature, if he will let you, and normal temp. for cats is 100.5 - 102.5
It's also possible that he was bitten by a venomous insect, spider or snake. He could have experienced a seizure or he may have an abnormality in his brain which is causing these symptoms. As you can imagine, these are only a few of the possibilities which could be causing his symptoms. In order to get a definite diagnosis and treatment (if needed) it would be advisable to have him evaluated by a vet in person as soon as possible.
Because he is a senior kitty, he is more vulnerable to develop underlying medical conditions. A routine senior blood test, including a thyroid panel, plus a urinalysis, will provide more diagnostic tools for the vet.
Due to his advanced age, it would be best to have him seen tonight, if possible. If your regular vet doesn't have emergency hours, bring him to the nearest ER vet center. If you don't know the location of any, please provide your city, state and zipcode and I will be glad to help research it for you.
I hope all will be well with Shred! Please let me know how he's doing and what the vet tells you. Thanks!
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