I have had the same family doctor for 25 years. On my last visit, I had been to my therapist, had a referral to an endo doc, and with my hair being 9 months long and with other changes in the works, even if subtle, I felt he needed to be aware of my transgenderness. He didn't blink an eye. When I asked if he had other transgendered patients he said no, just several with gender issues. I thought, that's me, I have gender issues - I was issued the wrong body!
I've got him in contact with my therapist and my endo doc. I go in again next month; I'm going to have to guide him in bringing his office up to speed (proper name, pronouns, dress change, etc.)
Anyway, to the original question - I just told him. If he hasn't heard everything, he's certainly heard an awful lot. Your doctor is a member of your medical team, she/he needs to know.
SusanKG