Hair biopsy guaranteed to determine?

Hello, I went to a dermatologist for my hair loss (that I've noticed for a few months now) and she had me have some blood tests. My Vitamin d is low being 28 out of the normal range of 30-100.That was the only thing that came abnormal while all the other things were fine.So now she mentioned that the next step is to have hair biopsy....

Also I took and still taking birth control pill named Errin for me to stop having periods. Now both of these; taking the pill and Vitamin D can cause the hair loss. But my question is, will the hair biopsy be actually able to tell me that the cause is from either of the two or even something else?
I just want to know for sure if the biopsy will be able to determine for sure otherwise I don't want to go through it for nothing. by the way will the hair that the test is done at grow back? Tweet