I can't afford blood testing for it at the moment, but I'm taking 10,000 IU a day anyway now. Especially for winter. According to my reading, sometimes absorption can be so poor in people that a dose sometimes much HIGHER than that is required as a loading dose to raise your levels. Just to be safe I'm sticking with the 10,000 IU. It's already much higher than the "recommended dose", but I don't care what Doctors "recommend". F--- 'em.