I call both my T and my pdoc by their first names. I've been with this mental health organization for a few years now at three different locations. At the other two locations, I had to call the pdoc "dr Surname". But at this location, which does both inpatient, residential, PHP and outpatient, it's "FirstName".

In my country you have two forms of "you", though, and I always use the most polite and distant form for my pdoc. Although he did say last session or the session before that that by now I should feel free to call him the other form.

Oh, and I once told my pdoc he wasn't a doctor but a pill therapist. He did NOT like that. A nurse was with me at the appointment and helped me explain that it was just about the biggest compliment I could give him - I'm terribly afraid of doctors and don't trust them (I'm a survivor of medical torture, so it's somewhat understandable) and saying he wasn't a doctor was just about saying "I recognize you're not evil".

Edit: I just turned 18, by the way. And I have (and have always had) therapy at a organisation for child and teen psychiatry. Don't know if that matters.

With my first two therapists, I always used their full name (first and last) when writing down their name. And to avoid being uncomfortable or awkward I started emails with just "Hi".