The vitamin D 1, 25 test is useless.
The important test is the D 25 hydroxy. This measures storage of vitamin D. If it is below 32 then you are deficient. If it is 8 or less you are severely deficient.
Vitamin D deficiency is definitely related to cancer and now suspected to also be related to autoimmune diseases.
Further, this is what your endocrinologist knows. Anytime you become even mildly vitamin D deficient, you run the risk of your parathyroid glands going into overdrive. These glands, not to be confused with the thyroid, regulate calcium in your body.
Low D 25 hydroxy can cause PTH, a measure of the parathyorid gland, and vitamin d 1, 25, a measure of serum vitamin D, to skyrocket. What is happening is that the parathyroid gland is desperately trying to keep calcium level in the bloodstream, despite lack of vitamin D.
The end result is the parathyroid glands can become exhausted. Eventually, could require surgery to remove them. Also, because these glands will leach calcium from the bones if they have to, to keep calcium in the blood constant, there is a higher risk of osteoporosis.
I have yet to meet a GI who understands any of the above.