The role of heavy metals in thyroid cancer: a meta-analysis

Abstract

BACKGROUND AND AIM: Although trace amounts of some metals are essential, high concentrations may be toxic and several metals have even been classified as carcinogenic by the International Agency for Research on Cancer. Certain metals also have endocrine disrupting properties and may therefore affect thyroid function. The objective of current study was to determine the possible association between metal levels in tissue or serum and thyroid cancer by performing a meta-analysis. METHODS: A systematic literature review was performed using the search terms thyroid cancer and metal(s), searching PubMed, Embase and Scopus through February 2021, and reviewing reference lists of all previously published relevant reviews. Articles were included if mean levels of metals in blood or tissue were reported or could be calculated and at least one comparison group was included (healthy or benign controls). Summary estimates were calculated using a random effects model including the weighted standardized mean difference (SMD) for each comparison. RESULTS:The search returned 9416 studies of which 125 studies were full-text reviewed and eventually 21 studies were included. No significant difference in blood levels between thyroid cancer patients and healthy controls was found for cadmium (SMD: 1.14; 95% CI: -0.44; 2.72), lead (SMD: 1.65; 95% CI: -0.37; 3.68), or zinc (SMD: -0.29; 95% CI: -2.21; 1.63). No significant difference in tissue levels between thyroid cancer patients and benign controls was found for lead (SMD: -0.17; 95% CI: -2.01; 1.67) or zinc (SMD: -0.54; 95% CI: -2.08; 1.00). CONCLUSIONS:Although no significant difference was found, only limited, mainly cross-sectional studies compared blood or tissue metal levels between thyroid cancer patients and a control group. Large, prospective studies including multiple blood draws over time over needed to better understand the risks of heavy metal exposure on the thyroid gland, especially given their potential carcinogenic and endocrine disrupting properties. KEYWORDS: Heavy metals, cancer and cancer precursors