If it isn't a doctor, it's by their first name, because that is what they call themselves. If they called themselves something else, that is what I would call them. I have never had a doctor ask me to call them by their first name and that's fine with me, even though they always call me by mine. I know that bugs some people but I couldn't care less. I don't want to call my doctor Jeff and I don't want him to call e Mrs. Bijou.
I actually got in trouble over this one once when I was working in a doctor's office. If a doctor wants to be on a last name basis with me, that's cool. I'm Mrs. Splash. But I do not wish to be Tiger to him/her while they are Dr. Name. Either we are both on a first name basis or a last name basis. I was comfortable going by last names for the entire office because of my military background, but did not like how the doctors (and some PAs) insisted on being called by their surnames while using my first name.