Warning for others: HN seems to be pro-armchair diagnosis on vitamin D supplements, which makes sense given the demographic and stereotypical lifestyle factors. Vitamin D is fat soluble. It can build up in your system over time. Normal levels are 30-50 ng/ml, and you can get your levels tested by asking for the test whenever you get a blood draw or your physical. Taking vitamin D supplements at high levels indefinitely without medical testing is dangerous (albeit a slowly building danger over months that is relatively easily reversed).

Anecdata: I was taking 5000 IU daily for a year, and my levels went up to 79 ng/ml. I stopped taking it for 6 months, and my levels went down to 47 ng/ml.