The role of vitamin D as an immune modulator has been emphasized in recent years, and low levels of the hormone were observed in several autoimmune diseases including multiple sclerosis and systemic lupus erythematosus. Vitamin D mediates its effect though binding to vitamin D receptor (VDR), and activation of VDR-responsive genes. While VDR gene polymorphism was found to associate with autoimmune thyroid diseases (AITDs), few studies examined levels of vitamin D in these patients and those that did yielded conflicting results. We therefore undertook to evaluate the levels of vitamin D in patients with AITDs compared to patients with non-AITDs and healthy controls. Serum vitamin D (25-OH) levels were measured in 50 patients with AITDs, 42 patients with non-AITDs and 98 healthy subjects, utilizing the LIAISON chemiluminescence immunoassay (DiaSorin, Saluggia, Italy). Vitamin D deficiency was designated at levels lower than 10 ng/ml. Antithyroid antibodies, thyroid functions and demographic parameters were evaluated in all patients. The prevalence of vitamin D deficiency was significantly higher in patients with AITDs compared with healthy individuals (72% versus 30.6%; P<0.001), as well as in patients with Hashimoto's thyroiditis compared to patients with non-AITDs (79% versus 52%; P<0.05). Vitamin D deficiency also correlated to the presence of antithyroid antibodies (P=0.01) and abnormal thyroid function tests (P=0.059). Significantly low levels of vitamin D were documented in patients with AITDs that were related to the presence of anti thyroid antibodies and abnormal thyroid function tests, suggesting the involvement of vitamin D in the pathogenesis of AITDs and the advisability of supplementation.