I was deemed to be vitamin D deficient in 2014 and given the 20,000 IU twice a week for 7 weeks, then a gap then a retest and after that I was prescribed 1x 800IU a day which I do take. In my case I had random symptoms like a specific kind of tinnitus and palpitations/balance weirdness which cleared up once I started supplements. I haven't been retested since pre-regular supplementation but part of that is the virtual impossibility of getting blood out of me and negative effects I get from trying...