Yes, it is okay. It has 75 mcg potassium Iodide per drop, so to get 400 mcg you're using 5-6 drops. Potassium Iodide is directly used by the thyroid to make thyroid hormone, so that is likely why it warms your fingers and toes.
It would be nice for you to avoid freezing fingers and toes. If you like, you can slowly add drops to be warm all the time.