My doctor told me that my hair loss was caused by Vitamin D deficiency a year ago. Since then I have been taking Vitamin D but the thinning hair I have did not get better.
A vitamin D deficiency can definitely cause hair loss. Treating it can stop the progression but it may not reverse it. You might have genetic hair loss as well and if you are a female you might have many other causes of hair loss which are discussed in this post from a previous baldingblog question posed back in 2009 which I outlined the approach a woman should do when they have hair loss: https://baldingblog.com/2009/03/13/woman-with-handfuls-of-hair-that-come-out-every-morning/