Email is best for an interest letter, phone call if you just want to check the status of your app and ask for info regarding upcoming dates of committee meetings and such. I agree with
@NonTradWarrior that you should only send an update letter if you have something substantial to add to your application (significant clinical experiences, recent publications, additional LORs, test scores, etc.), but anyone can send an interest letter! I've always felt that a combination update/interest letter is most effective, especially pre-interview when you may not have much to update them on, but you still want to express your continued interest in the school. It might be just the thing to rescue your application from ad com limbo! Just remember to keep it short (1 page or less) and to the point; anything longer and you risk boring the ad com readers.