For the first time in my life a doctor ordered a vitamin D test along with other blood labs. My D came out low, so I started taking D3. Vitamin D deficiency can affect mood, immune system function, and if it is severe vitamin D deficiency can cause bone disorders, such as rickets. No joke.
I had also noticed that my hair was coming out. My hair is thick, so the loss didn't "show" but when I combed it after washing there was way too much hair loss. My daughter had a vitamin D deficiency some years ago and also experienced hair loss - in fact, that was why she went to the doctor.
As for mood, I don't think I've noticed a change - but it's so hard to tell when I'm on several meds already.