When writing a letter to a doctor, use the doctor's full name, followed by MD in the address and salutation. It is generally better to send the address to the doctor's hospital or practice rather than to a personal address.
Most letters that patients write to doctors are formal in nature. In formal letters, the salutation is followed by a colon rather than a common. The letter should also be short and to the point, and what the sender wants or needs should be made clear in the first sentence. The letter should be signed with both a hand-written signature and a typed name.