Hi I Think My Child May Have Had Access To

Hi I think my child may have had access to a bottle of levothyroxine 120 mcg, he is 3 years old, 40 pounds if he took any it would have only been one. I have no idea if he did, this would have been anywhere from 4-6 hours ago. He has acted fine and like his normal self, I'm wondering if I should be concerned if he took any.
Thanks