Vitamin D levels are generally low in the dialysis population, and supplements are recommended to raise them. In 2007 Wolf and colleagues published in Kidney International ( Kidney Int. 2007 Oct;72(8):1004-13. Epub 2007 Aug 8.) that lower vitamin D levels were in untreated hemodialysis patients were associated with higher mortality.
Current recommendations are to raise the levels to at least above 30 ng/mL, and toxicity is very low with current forms recommended therapy. The reasons for supplementing vitamin D fall into two categories, a) classical - which deals with bone metabolism and in my opinion vascular calcification and cardiovascular complications, and b) non-classical - which deals with the effect of vitamin D on everything else - expecially inflammation and cancer prevention.
The question of vitamin D deficiency was looked at by a group from France, and published in J Nephrol. 2013 Jan-Feb;26(1):152-7. doi: 10.5301/jn.5000123.
They looked at 39 hemodialysis patients with vitamin D levels around 20 ng/mL, giving the supplement either in an oral dose of vitamin D2 (D2M) or D3 200,000 u per month (D3M), or D2 three times a week (D2s). Levels rose to over 30 in 84% of the D3 but only 15% of the D2M and 27% of D2S. Their observation was that D3 is superior to D2 at raising levels.
Here is an excellent review:
Vitamin D3 is also referred to as cholecalciferol and Vitamin D2 is referred to as ergocalciferol.