Titles (including the equivalents of mr and mrs) are never used in addressing people here, with a very few exceptions (members of the royal family, for instance). And we don't use people's names when we talk to them either, unless we need to indicate whom we are talking to. So I don't call my T anything, but if I did, it would only be possible for me to use his first name. In txts and letters, I open with "Dear [firstname]".

He is a psychiatrist, but that doesn't affect the address one way or the other.