Radioactive iodine treatment. is it ok to consume hemp protein shakes (containing zero sodium) during the the iodine free diet?

You need low iodine diet, not low sodium diet for preparation of RAI therapy. Please look at the label for iodine content if available. There is no need of vegan diet when you prepare for this treatment.

Supplements are not investigated by the FDA and there is no guarantee of the absence of contaminants including iodine. I don't recommend your drinking those shakes prior to radioiodine administration.