Exposure To Non-Persistent Pesticides And Thyroid Hormone Levels: A Systematic Review Of The Epidemiological Evidence

Abstract

Introduction: Numerous pesticides are recognized for their endocrine-disrupting properties. Non-persistent pesticides such as organophosphates (OP), carbamate and pyrethroids may interfere with thyroid function, as suggested by animal studies. However, how chronic exposure to these compounds affects human health is unknown. The present study aimed to review the epidemiological evidence for the association between exposure to non-persistent pesticides and thyroid hormone (TH) levels. Methods: A systematic review was conducted using MEDLINE and Virtual Health Library databases. Articles were limited to original studies and reports published in English, Portuguese or Spanish. Case-reports and studies only addressing organochlorine pesticides were excluded. Results: Seventeen epidemiological studies were identified that investigated the association between exposure to non-persistent pesticides and TH levels. The full text of one of them was not available. Of the remaining studies, 14 were cross-sectional, 11 were of occupationally exposed workers and 11 used exposure biomarkers. OP and metabolites were the most studied pesticides. Overall, pesticide exposure was not associated with TH levels. However, alterations compatible with hypothyroidism, i.e. reduced triiodotironine (T3) or thyroxin (T4), and elevated thyroid-stimulating hormone (TSH) were observed in 7 and 5 studies, respectively, whereas a few reports found positive association with T4 and negative association with TSH. Methodological heterogeneity between studies was noted, particularly regarding sample size, type of pesticide, exposure assessment method and statistical analysis. Conclusions: Current knowledge about the impact of non-persistent pesticides on human thyroid function is limited. In addition, the varying methodological approaches hamper drawing conclusions about the evidence on the topic. This review highlights the need for research to explore this relationship in different exposure scenarios.