Associations between cadmium and lead exposure and thyroid disorders: A systematic review and meta-analysis

Exposure to heavy metals, specifically Cadmium and lead, poses considerable public health risks, as these substances are known endocrine disruptors that can negatively influence the regulation of thyroid hormones. This systematic review and meta-analysis examine the associations between cadmium and lead exposure and thyroid disorders, including hypothyroidism, hyperthyroidism, and thyroid cancer. The study was conducted by searching PubMed, Scopus, Embase, Web of Science, and Google Scholar, identifying 23,333 studies. After removing duplicates, 4980 were excluded for reasons such as being reviews, non-English, or unrelated, and after assessing full-text, 21 studies were included for qualitative and quantitative analysis. A random-effects meta-analysis was conducted to estimate pooled odds ratios (95 % CI) for the association between heavy metal exposure and thyroid disorders, with heterogeneity assessed via the I² statistic. Meta-analysis confirmed significant associations between both metals and thyroid disorders. Lead exposure showed a strong relationship (pooled OR ≈ 2.7), while cadmium also demonstrated a meaningful association (pooled OR ≈ 1.6), though with moderate heterogeneity across studies, suggesting variations due to study design, exposure assessment, or demographic factors. These findings emphasize the public health importance of reducing exposure, as both metals possess endocrine-disrupting potential. Mechanistically, cadmium appears to act through oxidative and hormonal pathways, whereas lead shows stronger associations with thyroid cancer and autoimmunity, particularly in occupational settings. Limited evidence further indicates possible sex-related and gene-environment interactions. The evidence reviewed indicates that cadmium and lead may contribute to the development of thyroid disorders through various biological pathways.