Vitamin and mineral supplements and thyroid cancer: a systematic review

The purpose of this study was to consolidate epidemiological evidence for the association between dietary supplements of vitamins and minerals and thyroid cancer development, as well as to contribute to evidence-based dietary recommendations for thyroid cancer primary prevention. We carried out a systematic literature review specifically for dietary supplement and thyroid cancer risk. MEDLINE, EMBASE, and Dissertations and Theses were systematically searched to identify original epidemiological studies with a comparison group that investigated vitamin or mineral supplementation as an etiological factor for thyroid cancer. In total, 11 independent studies were identified and reviewed. Our qualitative summary showed conflicting results for common antioxidants including vitamins A, C, and E and β-carotene in relation to thyroid cancer. Similarly, results for dietary supplement combinations as well as other individual vitamins and minerals (vitamin B complex, vitamin D, iodine, calcium, zinc, magnesium, and iron) are largely inconsistent across studies. Overall, our review suggested that the current evidence to support any protective or hazardous effect of vitamin or mineral supplements on thyroid cancer development is inconclusive and additional studies addressing previous limitations are necessary to elucidate this possible association. In particular, reverse causality is of major concern and should be addressed by prospective studies with large and representative samples.