Etiquette is important. It shows that you take the time to do what is polite. You consider the formalities of respect, and you think of how the other person will feel upon receiving a politely addressed letter. When you are writing to your doctor (or a friend who happens to be a doctor), remember that doctors are addressed differently on envelopes than other people. It is important to address the envelope correctly, whether it is for a formal invitation or a mere thank you note.
Address the envelope as follows: if the husband is a doctor and the wife is not a doctor: Dr. and Mrs.(follow it by the husband's first and last name). If the husband's name is Tom Jones, the envelope should read Dr. and Mrs. Tom Jones.
Address the envelope as follows if both the husband and wife are doctors. If the husband's name is Tom Jones and the wife's name is Susan Jones, the envelope should read: Drs. Tom and Susan Jones.
Address the envelope as follows if both the husband and wife are doctors and have different last names. If the husband's name is Tom Jones and the wife's name is Susan Smith, the envelope should read:
Dr. Tom Jones
Dr. Susan Smith
Choose the most formal way to address a letter when in doubt.
If the couple has children and you want to include them on the envelope add the words "and Family" at the end of the line. So for example, Dr. and Mrs. Tom Jones and Family.
Don't limit this to medical doctors. Remember that some professionals other than medical doctors should be addressed as doctor. For example, people with their Ph.D. should be addressed as doctor. Some will think it is pretentious, but most will appreciate the extra respect.