All I needed to start HRT was a referral from my primary care physician to an endocrinologist, and neither my PCP nor my endo required a note from my therapist or informed consent. (This could be because of the doctors I see, though - both are supportive and have other trans patients.)
It's unfortunate how much gatekeeping is in place on trans healthcare. On paper and at first glance it can sound like a good idea, but when you think about all the other medical procedures and medications that are handed out without all this gatekeeping it increasingly only seems to be explainable as transphobic.