Many people experience hair loss or thinning hair. The cause of that hair loss may never be known well. In other cases, doctors can work with you to pinpoint the underlying cause of your hair loss. When that happens, you may be able to treat it and turn things around to allow your hair to resume its beautiful thickness and growth. One of the first areas to consider is Vitamin D. Chances are you may not be getting enough. This is the case for most Americans.