Thyroid Hormones in Conventional and Organic Farmers in Thailand

Abstract

Pesticides can act as endocrine disruptors by different mechanisms including inhibition of iodine absorption, increases in thyroid hormone clearance, decreased cellular uptake of thyroid hormones, or changes in expression of thyroid hormone regulated genes. This study examined how exposure to pesticides impacts thyroid hormone levels, thyroid stimulating hormone (TSH), triiodothyronine (T3), thyroxine (T4), free T3 (FT3), and free T4 (FT4) by comparing conventional (n = 195) and organic farmers (n = 222), and by evaluating which types of pesticides might be associated with changes in thyroid hormone levels. Questionnaires were used to collect information about farmer characteristics, self-reported stress, agricultural activities, and history of pesticide use. Conventional farmers were asked to report the type and quantity of pesticides used each day. The TSH, FT3, T3, and T4 levels of conventional farmers were 1.6, 1.2, 1.3, and 1.1 times higher than those of organic farmers, respectively, after adjusting for covariates. Several specific herbicides had a significant relationship between the amount applied and an increase in thyroid hormone levels, after covariate adjustment. They included: paraquat (TSH, FT3 and T3); acetochlor (FT4); atrazine (TSH, FT3 and T3); glyphosate (T4); diuron (TSH) and the “other” herbicides including alachlor, propanil, and butachlor (FT4 and T3). The most commonly used herbicide among conventional farmers was glyphosate, followed by paraquat, 2,4-dichlorophenoxyacetic acid (2,4-D). These findings suggest that exposure to pesticides could impact the development of metabolic diseases and other health outcomes by altering the endocrine system (the thyroid hormone levels) through the hypothalamic–pituitary–thyroid (HPT) axis. This work is a part of a longitudinal study which will evaluate the sub-chronic effects of repeated exposure to different types of pesticides on thyroid hormone levels.

Acute Changes in Thyroid Hormone Levels among Thai Pesticide Sprayers

“…All samples were analyzed at Buddhachinaraj Hospital, the regional medical center in Phitsanulok province using standard clinical laboratory methods. Analysis of thyroid hormones using a paramagnetic particle, chemiluminescent immunoassay for the quantitative determination of TSH, FT3, free thyroxine (FT4), total triiodothyronine (T3), and total thyroxine (T4) in human serum using the UniCel DxI 800 Access Immunoassay System, Beckman Coulter (Atlanta, GA, USA) [ 12 ]. The limit of detection was 0.005 μIU/mL for TSH, 0.09 ng/dL for FT3, 0.15 ng/dL for FT4, 0.01 μg/dL for T3 and 0.5 μg/dL for T4.…”

“…Additional covariates for each model were considered based on their significance in univariate analyses. In our previous cross-sectional study, we included gender, current smoking, current alcohol use, insecticide use at home in the past year, triglyceride levels and any stress symptoms in the past 2–4 weeks [ 12 ], so we examined these covariates in our univariate models. Others have also included socioeconomic status [ 46 , 47 ] in their work, so we also examined education and debt status in univariate models.…”

“…Several epidemiologic studies have established an association between OPs and alterations of thyroid hormone levels [ 8 , 9 ]. A limited number of studies have examined whether ever using OP pesticides was associated with hypothyroidism [ 9 , 10 , 11 ], or whether use in the past growing season was related to alterations in thyroid hormone levels [ 12 , 13 , 14 ]. Experimental animal studies have also shown that organophosphate (OP) disturb thyroid gland function [ 15 , 16 ].…”

“…We have previously shown in the larger longitudinal cohort, from which the farmers in this study were recruited, that after adjusting for covariates, thyroid hormone levels of conventional pesticide using farmers were significantly higher than those of organic farmers. Several specific herbicides had a significant relationship between the amount applied and an increase in thyroid hormone levels [ 12 ], and increasing the cumulative number of spray days of pesticides significantly increases the level of thyroid stimulating hormone (TSH) and free triiodothyronine (FT3) [ 13 ]. These farmers also had significantly higher abnormal metabolic biomarkers, body mass index (BMI), waist circumference, % body fat, triglycerides, total cholesterol, and low-density lipoprotein (LDL) values) compared to the organic farmers [ 36 ], and there was a significant association between the number of days of pesticide spraying and levels of cholesterol, high-density lipoprotein (HDL), LDL, blood pressure and BMI in the longitudinal cohort [ 37 ].…”

Longitudinal Study of Thyroid Hormones between Conventional and Organic Farmers in Thailand

“…All samples were analyzed at Buddhachinaraj Hospital, the regional medical center in Phitsanulok province, using standard clinical laboratory methods. The analysis of TSH, T3, T4, FT3, and FT4 was conducted using a paramagnetic particle, chemiluminescent immunoassay in human serum (UniCel DxI 800 Access Immunoassay System, Beckman Coulter, Atlanta, GA, USA) [ 12 ]. The limit of detection was 0.005 μIU/mL for TSH, 0.09 ng/dL for FT3, 0.15 ng/dL for FT4, 0.10 ng/mL for T3 and 0.50 μg/dL for T4.…”