Like all the rest, foods/drinks (and coffee), vitamins, and other drugs, take Welchol
hours away from your thyroid medication
can make it harder for your body to absorb certain other medications, including levothyroxine/Synthroid. So as long as you take them both far apart from each other, you should be fine.
Low Vitamin D, did you tell the doctor? Will you be taking supplements to build up your low V-D, I presume.