In one study, researchers found markedly low (<10) vitamin D levels in 48% of patients with Hashimoto's thyroiditis. While this study is on patients with hypothyroidism, the association between low levels of vitamin D and the production of thyroid antibodies may contribute to the disease mechanism in Graves' disease. While more studies are needed, it's important for everyone to have their levels of 25(OH) vitamin D tested and make sure their levels are optimal.