Toothbrushes might not seem like obvious hazards, but they should be treated with the same kind of caution as laptops or smartphones when boarding an airplane. If the lithium-ion-powered device happened to overheat — or worse — while in your checked bag, there would be no way to know while in the air. This is a much greater threat than it would be in your carry-on, where at least the flight crew can keep an eye on things. Power banks and laptops have been the most common to catch fire in recent years, but anything with a lithium power source carries at least some risk. It may be better to just keep it accessible in your carry-on.