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Hello and thank you for asking your question here. I haven't heard much about coconut oil being used in cats but the thoughts with coconut oil is that it helps those with low thyroid function which is the opposite in your situation. A normal heart rate for cats is 160-220 so I would be concerned if you are getting in the 140's consistently that there could be something else going on. It does sound like he is generally doing well so just keep an eye on things and do what you are doing. Based on his last numbers though have you considered mixing the EVO with the Y/D? This might be a good option for you at this time. In another month or if hyperthyroid symptoms develop another T4 check would be ideal.
If by chance you would have to have him go back on thyroid medications there are other options where the medication can be compounded and made into a topical which is rubbed onto the inside of the ear. No more dealing with oral medication.
Be sure your vet is also keeping an eye on his kidney functions as well.