My endocrinologist also gave me my prescriptions and ordered blood tests at our first meeting. I had very specific instructions on what to take first, how much, and when to start other prescribed items.
The test results came in before I had started some things, and when she had the results and saw a problem, she contacted me (via a messaging/e-mail system) and adjusted what I would be taking, along with scheduling some follow up tests and actions.
You'd probably need the letter before tests to demonstrate that the tests were medically necessary and get insurance to pay for them. It is probably best to wait for the endocrinologist to order what she really needs.