Thanks for posting your question. I am not a doctor, so I just wanted to make sure I meet your expectations in terms of my professional knowledge and insights. I am a dietitian, so I am happy to give advice on nutritional matters. Low levels of Vitamin D is more common than previously thought. Possible contributing factors (other than limited sunlight exposure that is not relevant to you). Low dietary intake may be a contributing factor - if your diet is low in vitamin D rich foods such as oily fish, eggs, full cream dairy, etc. Being overweight may result in lower circulating levels of vitamin D.  Some people naturally have a lowered absorption of vitamin D in their digestive system, notably with digestive system diseases such as Chron's disease. In some cases, the kidneys are not able to effectively convert vitamin D to its active form. If your low levels persist despite supplementation, you can discuss with your doctor the tests required to assess the roles of these possible contributing factors. Your doctor made a good recommendation. I would advise to have your blood levels tested again after 3-6 months.