Alice6065 is correct in that if you live in the northern part of the country we only get the rays that convert cholesterol to vitamin D from May to early fall.
Four years ago I had extremely low levels and was put on a prescription of 50,000 IU per week for one month, then 30,000 and finally down to anywhere from 2000 IU to 4000 IU per day. They now say that even people in the south are deficient because of sunscreens. Most doctors now do a D level check with an annual physical. When you take Vitamin D make sure it is D3 not D2.