Nutritional deficiencies are rarely a cause of hair loss despite the marketing of a wide variety of nutritional supplements that claim to somehow enhance hair growth or hair health. Of the possible nutritional deficiencies that can cause thinning hair, iron deficiency anemia is most common, and when it occurs it is more frequently seen in women. Iron deficiency anemia is a result of a decreased amount of red blood cells in the blood because of inadequate iron reserves in the body.