I've been taking 6,000IU/day of D3 for several years now. I had my D level checked last month and it was 62.4 ng/ml, which is within my understanding of the recommended range of 50-80 ng/ml. It seems likely to me that I would be deficient if I wasn't supplementing.

47 yr old male non-smoker, social drinker, fit. Jan'10, Stg3 rt tonsil+rt neck SCC, HPV+, rad+chmo Vancouver Cda. 2yr clear Apr'12 London UK. Apr'13 mets recur to lymph btw left lung & aorta, 3x Cisplatin+5FUchemo+20 rad, was all clear but 6-mo PET-CT shows mets to pleura around left lung, participating in St 1 trial of GDC-0980. GDC lost effect and ended July'14, bad atrial fibrillation requiring hospitalisation, start more standard chemo 10 Sep 2014.
Sadly has passed away, notified Jan 2015.