Vitamin D Deficiency is being linked to a myriad of health problems including Cancer and Autoimmune disorders like Diabetes, Multiple Sclerosis and Rheumatoid Arthritis.How do I know if I have Vitamin D Deficiency?Vitamin D level is determined by a blood test.The only way to know your level is to ask for a test called 25-hydroxyvitamin D or 25(OH)D.