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

Area of Science:

• Endocrinology
• Environmental Health
• Toxicology

Background:

• Chronic lead (Pb) exposure is recognized as an endocrine disruptor, potentially affecting various hormonal systems.
• The association between long-term lead exposure and thyroid hormone levels remains unclear, with conflicting reports in existing literature.

Purpose of the Study:

• To systematically evaluate the association between chronic lead exposure and thyroid function parameters.
• To synthesize findings from observational studies using meta-analysis to clarify the impact of lead on thyroid hormones.

Main Methods:

• A systematic review and meta-analysis of observational studies identified through comprehensive database searches (PubMed-Medline, Scopus, Embase) up to March 31, 2022.
• Studies were assessed for bias using the Newcastle Ottawa Scale, and heterogeneity was evaluated with the I² statistic.
• Random-effects modeling was employed to analyze mean differences in thyroid function parameters between lead-exposed and control groups.

Main Results:

• Fifteen observational studies were included, revealing a higher mean blood lead level in exposed individuals (28.07 µg/dl).
• Significantly elevated free T3 levels (0.48 pg/dl) and a trend towards higher T3, T4, FT4, and TSH were observed in the lead-exposed group.
• High heterogeneity (I² > 85%) was noted across the included studies.

Conclusions:

• Chronic lead exposure is associated with altered thyroid function, notably increased free T3 levels.
• The significant heterogeneity underscores the need for high-quality research to further elucidate the relationship between cumulative lead exposure and thyroid hormone levels.