I have been treated for hypothyroidism for the past 21 years. That also lowers vitamin D levels, because the thyroid hormone helps the body absorb vitamin D. I checked with google, and it says people with PMR and GCA are more likely to have Hashimoto's hypothyroidism than other people, because Hashimoto's is another auto-immune disease. So in addition to having your vitamin D checked, also have your thyroid checked if you haven't already.