You can get an inexpensive spot blood test to see if your D3 levels are optimum at 70 to 80 ng/ml. With 20,000 IU daily I'm sure you are spot on. http://tinyurl.com/Vitamin-D-Report
Taking 10,000 IU D3 daily put an end to my annual SEVERE chest colds that I had been suffering for DECADES ! Also ended getting the Flu. I refuse to take flu shots.