Correspondence: Muyesser Sayki Arslan MD, Diskapi Teaching and Research Hospital, Ankara, Turkey. 152 muyessersaykib@yahoo.com.tr Introduction Thyroid cancer is the most common endocrine cancer and its incidence is on the rise throughout the world (1-3). It is 2-4 times more frequent in females than in males but the incidence is similar between genders before puberty and after the female menopause (3). The vast majority of thyroid cancer is differentiated thyroid cancer (85% papillary cancer, 10% follicular cancer). Anaplastic and medullary thyroid carcinomas are seen rarely. The main reported causes of thyroid cancer is radioactive iodine exposure, previous childhood radiation to the head and neck, female gender, family history of thyroid cancer and iodine intake status (1,4). Accumulating evidence sourced from observational, epidemiological and preclinical studies showed that vitamin D deficiency increases the risk of developing many extraskeletal diseases including multiple malignancies. Among malignancies, colorectal cancer has the most clear association with circulating vitamin D levels (5). According to few data conducted on relationship between vitamin D deficiency and thyroid cancer and data from these studies had controversial results. Vitamin D is the precursor of the steroid hormone calcitriol (also called as 1,25-dihydroxy-vitamin D3 (1,25(OH)2D3) that mediates several actions in many cells of the body (6). CYP24A1 (24-hydroxylase) and CYP27B1 are cytochrome enzymes that have a role in steps of yielding calcitriol from vitamin D. The circulating form of vitamin D is 25(OH)D3 and its level in the blood is used for monitoring the vitamin D status of a patient. The defined levels for deficiency is set 75 nmol/L (30 ng/ml) by the Endocrine Society and 50 nmol/L (20ng/ml) by the Institute of Medicine (7,8). People have to receive adequate sunlight in order to maintain adequate vitamin D levels. However, this is not possible for many people due to various life conditions. Therefore, vitamin D deficiency is a very common condition throughout the world. Vitamin D might be one of the undetermined etiology leading to an increase of thyroid cancer prevalence in addition to the increased imaging of the neck. In this review, we focused on the studies about vitamin D and thyroid cancer to elucidate the role of vitamin D in the development, progression, prognosis and treatment of thyroid cancer. Calcitriol and cancer development Calcitriol is the predominant active metabolite of vitamin D and initiates or suppresses gene transcription by binding to the nuclear vitamin D receptor (VDR) (9). VDR is found in both the cytoplasm and nucleus of the most cells of the body but the expression level sometimes can be at low concentration (10,11). Calcitriol regulates indirectly or directly the human genome and induces CYP24A1 (6). Current data have been demonstrated that calcitriol diminishes tumor development through effects on cell proliferation, apopitosis, differentiation, inflammation, invasion, metastases, and angiogenesis (6). Calcitriol prevents the transition from the gap 1 (G1) to the synthesis (S) of the cycle. Known antiproliferative effects are increase in p21 and p27 expression and Review Article Acta Medica Anatolia Volume 2 Issue 4 2014 Vitamin D and Thyroid Cancer Muyesser Sayki Arslan 1 , Mustafa Sahin 2 , Tuncay Delibasi 1,3 1 Department of Endocrinology and Metabolism, Diskapi Teaching and Research Hospital, Ankara, Turkey 2 Department of Endocrinology and Metabolism, Ankara University School of Medicine, Ankara, Turkey 3 Department of Internal Medicine, Hacettepe University School of Medicine (Kastamonu), Ankara, Turkey Abstract Accumulating evidence demonstrated that the active form of vitamin D, 1,25(OH)2D3, has antiproliferative, anti-apopitotic and prodifferentiating effects in several tumour types in preclinical studies. Several studies reported the impact of vitamin D on cancer risk particularly in breast and colorectal cancer, however, its effect on thyroid cancer is less known. This review focuses on the relationship of vitamin D and thyroid cancer under the light of the literature. Thyroid cancer is the most common endocrine cancer and also vitamin D deficiency is a common condition throughout the world. Some clinical studies showed that vitamin D deficiency is higher in patients with thyroid cancer. Preclinical studies evidenced that vitamin D has an effect on differentiation, reduction in tumor burden, and prevention of metastatic growth in thyroid cancer used alone or in combination with anticancer drugs. However, further clinical studies are needed to understand its impact on prognosis of thyroid cancer. Keywords: Vitamin D, thyroid cancer, calcitriol. Received: 02.12.2014 Accepted: 29.12.2014 doi: 10.15824/actamedica.49519