Changes in thyroid status of rats after prolonged exposure to low dose dichlorodiphenyltrichloroethane

Exposure to pesticides and the risk of hypothyroidism: a systematic review and meta-analysis

BACKGROUND

Knowledge surrounding the association between exposure to pesticides and hypothyroidism is inconsistent and controversial.

METHODS

The aim of present study was, therefore, to review scientific evidence systematically and conduct a meta-analysis into the contribution of exposure to pesticides to hypothyroidism. PubMed, Scopus, Web of Science, and Google Scholar were searched. The findings are presented as OR, HR, PR, IRR, and 95% confidence interval (95%CI). A fixed-effect model using the inverse-variance method and random-effects inverse-variance model with DerSimonian-Laird method were used for estimating the pooled estimates. Cochran Q and I2 tests were used to confirm the heterogeneity of selected studies.

RESULTS

Twelve studies were included in the systematic review, and 9 studies in the meta-analysis. Epidemiological evidence suggested that exposure to insecticides including organochlorines, organophosphates, and pyrethroids increased risk of hypothyroidism (adjusted odds ratio (aOR) = 1.23, 95%CI = 1.14, 1.33 for organochlorines, aOR = 1.12, 95%CI = 1.07, 1.17 for organophosphates, aOR = 1.15, 95%CI = 1.03, 1.28 for pyrethroids). Exposure to herbicides also increased risk of hypothyroidism (aOR = 1.06, 95%CI = 1.02, 1.10). However, exposure to fungicides and fumigants was not found to be associated with hypothyroidism.

CONCLUSION

To increase current knowledge and confirm evidence to date future research needs to center on large-scale longitudinal epidemiological and biological studies, examination of dose-response relationships, the controlling of relevant confounding variables, using standardized and high sensitivity tools, and investigating the effects of environmental exposure.

Relationship between thyroid hormone parameters and exposure to a mixture of organochlorine pesticides, mercury and nutrients in the cord blood of newborns

The fetus is prenatally exposed to a mixture of organochlorine pesticides (OCPs), mercury (Hg), docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA) and selenium (Se) through maternal seafood consumption in real-life scenario. Prenatal exposure to these contaminants and nutrients has been suggested to affect thyroid hormone (TH) status in newborns, but the potential relationships between them are unclear and the joint effects of the mixture are seldom analyzed. The aim of the study is to investigate the associations of prenatal exposure to a mixture of OCPs, Hg, DHA, EPA and Se with TH parameters in newborns. 228 mother-infant pairs in Shanghai, China were included. We measured 20 OCPs, total Hg, DHA, EPA and Se in cord blood samples as exposure variables. The total thyroxine (TT4), free thyroxine (FT4), total triiodothyronine (TT3), free triiodothyronine (FT3), and thyroid-stimulating hormone (TSH) levels and the FT3/FT4 ratio in cord serum were determined as outcomes. Using linear regression models, generalized additive models and Bayesian kernel machine regression, we found dose-response relationships of the mixture component with outcomes: among the contaminants, p,p'-DDE was the most important positive predictor of TT3, while HCB was predominantly positively associated with FT3 and the FT3/FT4 ratio, indicating different mechanisms underlying these relationships; among the nutrients, EPA was first found to be positively related to the FT3/FT4 ratio. Additionally, we found suggestive evidence of interactions between p,p'-DDE and HCB on both TT3 and FT3, and EPA by HCB interactions for TT3, FT3 and FT3/FT4 ratio. However, the overall effects of the mixture on thyroid hormone parameters were not significant. Our result suggests that prenatal exposure to p,p'-DDE, HCB and EPA as part of a mixture might affect thyroid function of newborns in independent and interactive ways. The potential biological mechanisms merit further investigation.

The association between prenatal exposure to organochlorine compounds and neonatal thyroid hormone levels: a systematic review

In this systematic review, the association between prenatal exposure to organochlorine pesticides (OCPs) and neonatal thyroid hormone levels was studied. A systematic search of scientific literature was performed from the PubMed, SCOPUS and ISI web of science electronic bibliographic databases. The search strategy for the review was [(organochlorine OR "organochlorine pesticides" OR "organochlorine pollutants" OR "organochlorine pollutant") AND ("thyroid hormone" OR triiodothyronine OR Thyroxine OR "fetal thyroid function" OR "thyroid function" OR "Thyroid Stimulating Hormone" AND "prenatal" AND "maternal exposure")] in English sources. In this review, 305 papers (PubMed: 30; Scopus: 29; ISI: 246) were identified through an electronic database search. Twenty-seven articles were assessed for eligibility, from which 16 qualified articles were selected for the final evaluation. The most common OCP metabolites which were evaluated in order were hexachlorobenzene (HCB) (13 studies), pp-dichlorodiphenyldichloroethylene (pp-DDE) (13 studies), hexachlorocyclohexane (HCH) (10 studies) and dichlorodiphenyltrichlorethane (DDT) (eight studies). A review of the documents related to the association of prenatal exposure of OCPs with fetal or neonatal thyroid function tests provides us with heterogeneous data in this field. Factors such as differences in the studied populations and their area, ethnic and genetic background, time and rate of exposure, possible interaction of other thyroid-disrupting environmental factors and dietary intake of micronutrients such as iodine and/or selenium are considered the main limitations for making an accurate conclusion. For some OCPs including DDT, DDE, HCH and HCB, there are supporting evidences, and it is suggested that their exposure could potentially alter the fetal thyroid function and consequently impair the neurodevelopment process of the infants.

Pesticides With Potential Thyroid Hormone-Disrupting Effects: A Review of Recent Data

Plant Protection Products, more commonly referred to as pesticides and biocides, are used to control a wide range of yield-reducing pests including insects, fungi, nematodes, and weeds. Concern has been raised that some pesticides may act as endocrine disrupting chemicals (EDCs) with the potential to interfere with the hormone systems of non-target invertebrates and vertebrates, including humans. EDCs act at low doses and particularly vulnerable periods of exposure include pre- and perinatal development. Of critical concern is the number of pesticides with the potential to interfere with the developing nervous system and brain, notably with thyroid hormone signaling. Across vertebrates, thyroid hormone orchestrates metamorphosis, brain development, and metabolism. Pesticide action on thyroid homeostasis can involve interference with TH production and its control, displacement from distributor proteins and liver metabolism. Here we focused on thyroid endpoints for each of the different classes of pesticides reviewing epidemiological and experimental studies carried out both in in vivo and in vitro. We conclude first, that many pesticides were placed on the market with insufficient testing, other than acute or chronic toxicity, and second, that thyroid-specific endpoints for neurodevelopmental effects and mixture assessment are largely absent from regulatory directives.

Morphological Changes in the Thymus of Newborn Rats Exposed to Endocrine Disruptor Dichlorodiphenyltrichloroethane (DDT) during the Prenatal Period

The structure of the thymus in newborn rats exposed in utero to endocrine disruptor DDT was studied. We found that exposure to low doses of DDT during pregnancy does not disturb thymus formation in the offspring, but leads to changes in its structure than manifested as slower lobulation, wider epithelium-free areas in the cortex, and lower number of thymic corpuscles in the medulla. The results attest to sensitivity of the reticuloepithelial stroma of the thymus to the effects of endocrine disruptor.

Thyroid Function Disruptors: from nature to chemicals

The modern concept of thyroid disruptors includes man-made chemicals and bioactive compounds from food that interfere with any aspect of the hypothalamus-pituitary-thyroid axis, thyroid hormone biosynthesis and secretion, blood and transmembrane transport, metabolism and local action of thyroid hormones. This review highlights relevant disruptors that effect populations through their diet: directly from food itself (fish oil and polyunsaturated fatty acids, pepper, coffee, cinnamon and resveratrol/grapes), through vegetable cultivation (pesticides) and from containers for food storage and cooking (bisphenol A, phthalates and polybrominated diphenyl ethers). Due to the vital role of thyroid hormones during every stage of life, we review effects from the gestational period through to adulthood, including evidence from in vitro studies, rodent models, human trials and epidemiological studies.

Nitrate: An Environmental Endocrine Disruptor? A Review of Evidence and Research Needs

Nitrate is heavily used as an agricultural fertilizer and is today a ubiquitous environmental pollutant. Environmental endocrine effects caused by nitrate have received increasing attention over the last 15 years. Nitrate is hypothesized to interfere with thyroid and steroid hormone homeostasis and developmental and reproductive end points. The current review focuses on aquatic ecotoxicology with emphasis on field and laboratory controlled in vitro and in vivo studies. Furthermore, nitrate is just one of several forms of nitrogen that is present in the environment and many of these are quickly interconvertible. Therefore, the focus is additionally confined to the oxidized nitrogen species (nitrate, nitrite and nitric oxide). We reviewed 26 environmental toxicology studies and our main findings are (1) nitrate has endocrine disrupting properties and hypotheses for mechanisms exist, which warrants for further investigations; (2) there are issues determining actual nitrate-speciation and abundance is not quantified in a number of studies, making links to speciation-specific effects difficult; and (3) more advanced analytical chemistry methodologies are needed both for exposure assessment and in the determination of endocrine biomarkers.

Thyroid-disrupting chemicals and brain development: an update

This review covers recent findings on the main categories of thyroid hormone-disrupting chemicals and their effects on brain development. We draw mostly on epidemiological and experimental data published in the last decade. For each chemical class considered, we deal with not only the thyroid hormone-disrupting effects but also briefly mention the main mechanisms by which the same chemicals could modify estrogen and/or androgen signalling, thereby exacerbating adverse effects on endocrine-dependent developmental programmes. Further, we emphasize recent data showing how maternal thyroid hormone signalling during early pregnancy affects not only offspring IQ, but also neurodevelopmental disease risk. These recent findings add to established knowledge on the crucial importance of iodine and thyroid hormone for optimal brain development. We propose that prenatal exposure to mixtures of thyroid hormone-disrupting chemicals provides a plausible biological mechanism contributing to current increases in the incidence of neurodevelopmental disease and IQ loss.

Primordial and Primary Preventions of Thyroid Disease

BACKGROUND

Primordial and primary preventions of thyroid diseases are concerned with avoiding the appearance of risk factors, delaying the progression to overt disease, and minimizing the impact of illness.

SUMMARY

Using related key words, 446 articles related to primordial and primary, preventions of thyroid diseases published between 2001-2015 were evaluated, categorized and analyzed. Prevention and elimination of iodine deficiency are major steps that have been successfully achieved and maintained in many countries of the world in last 2 decades. Recent investigations related to the effect of cigarette smoking, alcohol consumption, and autoimmunity in the prevention of thyroid disorders have been reviewed.

CONCLUSIONS

The cornerstone for successful prevention of thyroid disease entails timely implementation of its primordial and primary preventions, which must be highly prioritized in related health strategies by health authorities.

Effect of Prenatal and Postnatal Exposure to Low Doses of DDT on Catecholamine Secretion in Rats in Different Period of Ontogeny

We studied the effects of prenatal and postnatal exposure to low doses of DDT on secretion of basic catecholamines epinephrine and norepinephrine in pubertal and adult rats. It was found that the endocrine-disrupting chemical under study led to a progressive decrease in the content of epinephrine and especially norepinephrine in systemic circulation, which indicated their disturbed secretion by the adrenal medulla and sympathetic nervous system cells. In animals exposed to low doses of DDT in both pre- and postnatal periods, the decrease in catecholamine secretion after puberty was less pronounced than in animals exposed only during the postnatal period, which can indicate the development of compensatory processes.

Effect of exposure to p,p´-DDE during the first half of pregnancy in the maternal thyroid profile of female residents in a Mexican floriculture area

BACKGROUND

Dichlorodiphenyldichloroethene (p,p´-DDE), the main metabolite of dichlorodiphenyltrichloroethane (DDT), has been associated with changes in human thyroid hormone levels. Maternal thyroid hormones are essential for adequate fetal neurodevelopment during the first half of pregnancy.

OBJECTIVE

To evaluate the association between maternal p,p´-DDE concentration and the maternal thyroid profile during the first half of pregnancy.

MATERIALS AND METHODS

We analyzed the information of 430 pregnant women from a Mexican floriculture area, with a gestational age ≤16 weeks. By questionnaire, we obtained sociodemographic, reproductive, and life-style, information. Serum concentrations of thyroid stimulating hormone (TSH), and total and free T3 and T4 were determined by means of Enzyme-Linked ImmunoSorbent Assay (ELISA). p,p´-DDE was analyzed by Gas Chromatography. The association between p,p´-DDE and thyroid profile was assessed through linear and logistic regression models.

RESULTS

Thirty eight percent of women had p,p´-DDE levels below the Limit of Detection and 12.3% below the Limit of Quantification. Within the quantifiable range, median was 53.03ng/g. TSH >2.5 mIU/L was present in 9.3% of women; 47.7% had isolated hypothyroxinemia; 3.5% had subclinical hypothyroidism, and 5.8% had overt hypothyroidism. We observed a significant positive association between quantifiable p,p´-DDE and total T3 serum levels in comparison with those with concentrations below the Limit of Detection (β=0.19; 95% CI=0.06, 0.34). There were no significant associations with other hormones of the thyroid profile or with clinical diagnosis.

CONCLUSIONS

Our findings suggest that p,p´-DDE exposure, even at low concentrations, could disrupt thyroid homeostasis during pregnancy.

Developmental neurotoxicity of succeeding generations of insecticides

Insecticides are by design toxic. They must be toxic to effectively kill target species of insects. Unfortunately, they also have off-target toxic effects that can harm other species, including humans. Developmental neurotoxicity is one of the most prominent off-target toxic risks of insecticides. Over the past seven decades several classes of insecticides have been developed, each with their own mechanisms of effect and toxic side effects. This review covers the developmental neurotoxicity of the succeeding generations of insecticides including organochlorines, organophosphates, pyrethroids, carbamates and neonicotinoids. The goal of new insecticide development is to more effectively kill target species with fewer toxic side effects on non-target species. From the experience with the developmental neurotoxicity caused by the generations of insecticides developed in the past advice is offered how to proceed with future insecticide development to decrease neurotoxic risk.

Morphofunctional Changes in the Thyroid Gland of Pubertal and Postpubertal Rats Exposed to Low Dose of DDT

Consumption of low dose DDT from birth until puberty suppressed functional activity of the thyroid gland in rats and leads to destructive changes in the organ during the postpubertal period with compensatory stimulation of secretory activity of thyrocytes aimed at normalization of thyroxin production.

Mechanisms of Disruptive Action of Dichlorodiphenyltrichloroethane (DDT) on the Function of Thyroid Follicular Epitheliocytes

The mechanism of disruptive action of dichlorodiphenyltrichloroethane (DDT) on the function of thyroid follicular epitheliocytes was examined in mature male Wistar rats administered with a daily dose of DDT (2 μg/kg) for 6 or 10 weeks. In 6 weeks, the serum level of Na(+)/I(-) symporter responsible for the transport of iodides into follicular thyrocytes decreased, while serum concentration of thyroperoxidase increased. In 10 weeks, both the number of follicles in the thyroid gland and serum level of Na(+)/I(-) symporter increased. In circulating blood, the concentrations of thyroglobulin as well as α and β receptors of the thyroid hormones did not change. DDT down-regulated the iodine-accumulated function of follicular thyrocytes by suppressing Na(+)/I(-) symporter synthesis. Recovery of Na(+)/I (-) symporter production resulted from up-regulation of the proliferative processes in thyroid gland.