My cat accidentally ate dog's Thyrosyn. Should I be worried?

Most Levothyroxine tablets come in 0.1-0.9mg dosages, so I am not sure why your pet is on a full 5mg dosage. If this is not a typo, then this is a pretty hefty dosage for a cat to receive if she surely got 2.5mg's. However, severe life threatening symptoms from a single overdose are less likely than with chronic overdosing. Some signs you may see are: Vomiting, diarrhea, hyperactivity to lethargy, hypertension, tachycardia, tachypnea, dyspnea, and abnormal pupillary light reflexes. It can take 1-9 hrs for absorption to occur. Then the drug should be mostly out of her system 12-24 hrs. In most cases this is not an emergency situation, but you can see the symptoms I mentioned above, which should be treated with supportive care through your vet.

This is a tough one indeed, but it is likely that Kilala will be ok, since I assume that the dosage you gave has to be incorrect based on the typical dosage that dogs receive (between 0.1-0.9mg). I hope that everything works out well for your pet Kilala. Best wishes and take care!