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

Abstract

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.

Supplementary information: The online version contains supplementary material available at 10.1007/s12291-023-01113-8.

Keywords: Lead exposure; Systematic review; Thyroid function parameters; Thyroxine; Triiodothyronine.

Fig. 1

PRISMA flow chart. Legends PRISMA 2020 flow chart listing the number of citations included and excluded at various steps, leading to the final list of citations for data extraction and meta-analysis

Fig. 2

Forest plot for the association between occupational Pb exposure and Blood lead levels. Legends/footnotes Forest plot revealing the group mean differences of blood lead levels (BLL) in µg/dL between the occupationally Pb exposed workers and control participants (i.e. without obvious Pb exposure). The square and whisker (horizontal lines) represent respectively the mean difference and 95% confidence interval of individual studies. The length and width of the diamond indicate the pooled mean difference and 95% confidence interval derived from random-effect analysis

Fig. 3

Forest plot for the association between occupational Pb exposure and levels of thyroid-stimulating hormones (TSH). Legends/footnotes Forest plot revealing the group mean differences in levels of TSH in mIU/L between the occupationally Pb exposed workers and control participants. The square and whisker (horizontal lines) represent respectively the mean difference and 95% confidence interval of individual studies. The length and width of the diamond indicate respectively the pooled mean difference and 95% confidence interval derived from random-effect analysis

Fig. 4

Forest Plot for group differences in triiodothyronine (A) and thyroxine (B) between the occupationally Pb exposed and control workers. Legends/footnotes Forest plot revealing the group mean differences in triiodothyronine in ng/dL (A) and thyroxine in µg/dL (B) between the occupationally Pb exposed workers and control participants. The square and whisker (horizontal line) represent respectively the mean difference and 95% confidence interval of individual studies. The length and width of the diamond indicate respectively the pooled mean difference and 95% confidence interval derived from random-effect analysis

Fig. 5

Forest Plot for group differences in free-triiodothyronine (A) and free-thyroxine (B) between the occupationally Pb exposed and control workers. Legends/footnotes Forest plot revealing the group mean differences in free-triiodothyronine in ng/dL (A) and free-thyroxine in µg/dL (B) between the occupationally Pb exposed workers and control participants. The square and whisker (horizontal line) represent respectively the mean difference and 95% confidence interval of individual studies. The length and width of the diamond indicate respectively the pooled mean difference and 95% confidence interval derived from random-effect analysis