Vitamin D and thyroid disease: to D or not to D?

The main role of vitamin D is to maintain calcium and phosphorus homeostasis, thus preserving bone health. Recent evidence has demonstrated that vitamin D may also have a role in a variety of nonskeletal disorders such as endocrine diseases and in particular type 1 diabetes, type 2 diabetes, adrenal diseases and polycystic ovary syndrome. Low levels of vitamin D have also been associated with thyroid disease, such as Hashimoto's thyroiditis. Similarly, patients with new-onset Graves' disease were found to have decreased 25-hydroxyvitamin D concentrations. Impaired vitamin D signaling has been reported to encourage thyroid tumorigenesis. This review will focus on the role of vitamin D in thyroid diseases, both autoimmune diseases and thyroid cancer, and will summarize the results of vitamin D supplementation studies performed in patients with thyroid disorders. Although observational studies support a beneficial role of vitamin D in the management of thyroid disease, randomized controlled trials are required to provide insight into the efficacy and safety of vitamin D as a therapeutic tool for this dysfunction.