Can you practice intermitten fasting if you have an overactive thyroid?

Yes once controlled: Once your thyroid level is well controlled (within a normal range), then intermittent fasting would probably be okay. Of course, fasting can mean a lot of different things. I'm assuming you mean not eating anything for a period of time - this is generally safe for a day or so but for prolonged fasts - thyroid problems or not - you need to make sure you are fasting in a healthy way.

Yes, but. . .: If fasting is something someone already did regularly, they could continue doing so with mild hyperthyroidism. However, fasting would be inadvisable if they were losing weight due to the hyperthyroidism. And it would not do anything to help to fix the thyroid disorder. That would require surgery, radioactive iodine treatment, or thyroid blocking medication.