I have had a vitiman d3 deficiency for 4 years now what could be the cause.

Vitamin D deficiency: The causes may be a lack of sunlight exposure and/or medications, obesity & dietary deficiency. It is well documented that an array of illnesses may arise from Vitamin D deficiency. supplementing with vitamin D3 produces higher serum levels of Vitamin D and also helps reduce non-vertebral fractures. You should follow with your doctor for a plan and to ensure your levels are continually monitored.