The signs of vitamin D deficiency can be so subtle that many people don't realize they have one. While symptoms can vary, some of the potential signs of a vitamin D deficiency may include: - Getting sick more often. Vitamin D is heavily involved in your immune function and ability to fight off infections.  - Feeling more tired than usual. Vitamin D deficiency has been associated with fatigue for many people. - Experiencing new bone, muscle, or back pain. Having enough vitamin D is needed for calcium absorption and bone metabolism, and a deficiency may be linked to growing pains. - Feeling down. Vitamin D deficiency has been associated with an increased risk for depression, particularly among adults.  - Hair loss. Some studies have found an association between vitamin D deficiency and alopecia.