I can't explain your white blood cell changes, and probably the attending doctor can't explain you. The most reasonable reason is that your body is very sensitive to leukopenia drugs when you first take them.
You don't have to worry, just follow the doctor's advice. If necessary, the doctor will stop taking the medicine, let you stabilize the white blood cells first, and then advise you to have an operation if necessary. The doctor can still let you eat like this, because your body can eat like this for the time being.
Exercise is necessary, but don't do strenuous exercise. Hyperthyroidism has a fast heart rate and a heavy heart load. Can't do strenuous exercise.
In addition, be sure to monitor blood routine and liver function once a week and observe at any time. Be sure to show the doctor the results of recent times every time.