Association Between Blood Lead Levels and Thyroid Function: An Updated Systematic Review and Meta-Analysis

Chronic Lead (Pb) exposure is associated with disrupting certain endocrine levels and is referred to as an endocrine disruptor. Thyroid hormones, involved in regulating numerous physiological functions, are reported with conflicting associations with chronic Pb exposure. This study broadly evaluated the association between long-term lead exposure and thyroid function parameters. In this systematic review, the observational studies documenting the changes in thyroid function parameters between the chronically Pb-exposed and controls were systematically identified from PubMed-Medline, Scopus, and Embase digital databases from the beginning to March 31, 2022. The protocol was previously registered with PROSPERO (ID: CRD42022315520) and executed following PRISMA 2020 guidelines. The random-effects model was used to analyze the mean differences in thyroid function parameters between groups. The classical I2 statistic was applied to assess heterogeneity, while the Newcastle Ottawa Scale was used to assess the risk of various biases. Where data allowed, sub-group, sensitivity, and meta-regression analyses were carried out. The current systematic review included fifteen observational studies. The Pb-exposed have a higher mean blood Pb level [i.e. 28.07 (95% CI 21.43–34.72) µg/dl], significantly higher free T3 [(i.e. 0.48 pg/dl (95% CI 0.05–0.95)] and trend of higher T3. T4, FT4, and TSH levels than controls with high heterogeneity (I2 > 85%). Considering the important role of thyroid hormone in multiple biological functions, the present findings emphasize the requisite for high-quality studies to investigate the association between levels of thyroid function parameters among individuals known for cumulative exposure to Pb.