I believe that the current SOC requires one letter for top surgery. It could be from a therapist, psychologist, or psychiatrist. The expectation is that the person have some experience working with trans people, but I don't think that is a huge sticking point with most surgeons at this point.
For bottom surgery, you must have two letters.
A primary care doctor, or any physician that feels competent to do so, can approve "informed consent" for testosterone therapy. Prior to this current SOC (Sept 2012), it had to be a therapist/pscyhologist/psychiatrist who wrote a letter.