I've seen LCSWs in the past and always addressed them by their first names. My current T is a PsyD whom I've been seeing for like 3 months.

I don't remember if he introduced himself by name when we first met, and he's never had a reason to do so since, so I have no clue what he'd expect me to refer to him as.
I've continued to mentally refer to him as Dr. [Last Name]. I have no clue whether he'd think that that level of formality was weird given the conversations we have, or whether he'd think it was presumptuous of me to refer to him by his first name.
In our last appointment he was giving some hypothetical of someone saying something to him and used the shortened version of his first name, which caught me by surprise. I know his first name from his profile on the office website, but that's the full version of his first name and it didn't occur to me that he might go by that shortened version (like William to Will). I can't even imagine calling him that.
I'm in academia, and so I kind of go by those conventions (if someone outranks you in title or position then use the full title until they indicate otherwise, like signing an email with just their first name), but he's my therapist, not a work colleague, so I'm not sure what conventions apply.

Did your therapist tell you what to call them? Did you decide?