I take 4000 a day of D per instructions of my hematologist following my blood work a few years ago. I asked to have my D levels checked a couple of years ago and I was found to be low. My doctor recommended that I take 4,000 units a day and that brought up my levels. I think you have to ask to have it checked as it is not something that is routinely checked. I get my blood checked every 3-4 months for my thyroid level and once a year by a hematologist for a complete work up. He is the one who checked for the vitamin D levels and suggested the amount of supplement to take. Taking the 4000 per day puts me right in the middle of the standard range of 30 - 100 ng/mL I have copd and vitamin D is supposed to have a positive impact on respiratory function and helps to ward off respiratory infections.

Anyway.....I take 4,000 per day and have my levels monitored each year in my blood work.