When you look at the LinkedIn profile (or in any other profile) of a person you don't know and you would like to contact, if the person indicates in his/her education to have a PhD, this means that you should start your email with:
"Dear Doctor Last-Name or Dear Dr. Last-Name,"
Indeed, PhD means that the person followed a doctorate program. If the person is occupying a teaching position at a University, and if the teaching position is Professor or Associate-Professor, you should start your email with:
"Dear Professor Last-Name or Dear Prof. Last-Name,"
If the teaching position is Assistant-Professor, the etiquette is to start with: Dear Dr. Last-name (if of course, he/she holds a PhD)
The Doctorate is the highest degree delivered by a University recognized by its country (or state or county). And every people, in every disciplines, having succeed a doctorate program should be called Doctor.
The PhD is the Doctorate degree commonly delivered in soft or hard sciences (soft sciences = human sciences + social sciences / hard sciences = formal sciences + natural sciences). Even if the PhD is not a job position, but a degree, the etiquette is to call a PhD holder: Doctor.
In France, a journalist after an interview of an academic researcher in chemistry, mentioned the researcher name in the article with quotes "Doctor X" to indicate to readers that the researcher claims to be a "doctor" but in fact he is not a real doc (as physicians)! Of course, the researcher offended by the quotes, sued the journalist and Dr. Chemistry won, and got serious and published apologizes from the journal.
The etiquette explained above concerning starting your email with "Dear Dr. X," is also true for other kinds of Doctorate degrees:
- MD = Doctor in Medical Sciences (physicians hold a Medical Doctor degree)
- JD = Doctor in Juridical Sciences (some lawyers can hold a Juris Doctor degree)
- For a complete list of Doctorate degrees: http://en.wikipedia.org/wiki/Doctorate
The Doctorate degree in some healthcare fields is used as both a University degree and a professional title/job position. The best-known is in the medecine where MD = physician/surgeon (except in UK where surgeons are called Mr. Last Name), but also your dentist having a DDM (Doctor in Dental Medicine), your pharmacyst holding a PharmD (Doctor of Pharmacy), or your veterinarian holding a VMD (Doctor of Veterinary Medicine).