Effects of Environmental Synthetic Chemicals on Thyroid Function

Abstract

Synthetic chemicals are released into the environment by design (pesticides) or as a result of industrial activity. It is well known that natural environmental chemicals can cause goiter or thyroid imbalance. However, the effects of synthetic chemicals on thyroid function have received little attention, and there is much controversy over their potential clinical impact, because few studies have been conducted in humans. This article reviews the literature on possible thyroid disruption in wildlife, humans, and experimental animals and focuses on the most studied chemicals: the pesticides DDT, amitrole, and the thiocarbamate family, including ethylenethiourea, and the industrial chemicals polyhalogenated hydrocarbons, phenol derivatives, and phthalates. Wildlife observations in polluted areas clearly demonstrate a significant incidence of goiter and/or thyroid imbalance in several species. Experimental evidence in rodents, fish, and primates confirms the potentiality for thyroid disruption of several chemicals and illustrates the mechanisms involved. In adult humans, however, exposure to background levels of chemicals does not seem to have a significant negative effect on thyroid function, while exposure at higher levels, occupational or accidental, may produce mild thyroid changes. The impact of transgenerational, background exposure in utero on fetal neurodevelopment and later childhood cognitive function is now under scrutiny. There are several studies linking a lack of optimal neurological function in infants and children with high background levels of exposure to polychlorinated biphenyls (PCBs), dioxins, and/or co-contaminants, but it is unclear if the effects are caused by thyroid disruption in utero or direct neurotoxicity.

Cognitive effects of endocrine-disrupting chemicals in animals.

“…Subtle problems with hearing, speech, and word comprehension are also common. A large number of environmental chemicals are known or suspected of altering thyroid hormone function (8). Exposure to these chemicals during early development could potentially interfere with brain development and cause permanent deficits in cognitive function.…”

“…For the most part, these are compounds that have estrogenic or antiestrogenic actions (58) and/or disrupt thyroid function (8). A much smaller number of chemicals have been evaluated for effects on other endocrine systems such as androgens and adrenal steroids.…”

“…Disruption of the thyroid system during development has been shown to result in permanent changes in neurochemical (2), morphologic (3,4), and neurobehavioral end points (5)(6)(7). There are a number of environmental contaminants that have been shown to affect thyroid system function (8). One example is PCBs.…”

Effects of Pesticides on the Peripheral and Central Nervous System in Tobacco Farmers in Malaysia: Studies on Peripheral Nerve Conduction, Brain-Evoked Potentials and Computerized Posturography

“…The major occupational populations at risk are workers in agriculture 1) . Furthermore, pesticide residue 2) , carcinogenicity 3) , and endocrine-disrupting chemicals 3,4) have recently become major concerns in the world.…”

Developmental Exposure to 4-hydroxy-2,3,3',4',5-pentachlorobiphenyl (4-OH-CB107): Long-Term Effects on Brain Development, Behavior, and Brain Stem Auditory Evoked Potentials in Rats

“…These developmental effects include alterations in thyroid hormone homeostasis (Brouwer et al, 1998;Brucker-Davis, 1998;Morse et al, 1996a), neurobehavioral effects Schantz et al, 1995;Seo et al, 1995;Tilson and Kodavanti, 1997), reproductive and endocrine effects (Hany et al, 1999a;Peterson et al, 1993) and neurochemical effects (Morse et al, 1996b;Seegal, 1996). The adverse effects caused by PCBs are dependent on the time of exposure and the structural characteristics of the PCB congener.…”