Introduction
Substances in the environment, known as endocrine [ENdoh-krin] disruptors, can alter hormone function. Most research has focused on substances that affect reproductive [ree-pro-DUK-tiv] hormones. However, more than 100 natural and synthetic substances have effects on thyroid function. Because thyroid hormone is needed for the body to develop normally before birth and in early life, anything in the environment that may affect the thyroid is a major concern for pregnant women and infants.
Perchlorate
Perchlorate [per-KLOR-ate] is used in many things, such as rockets, fireworks, road flares, matches, and air bag systems. Some fertilizers contain perchlorate and low levels may also be found in the environment due to natural processes. Perchlorate is present in some drinking water in the United States and worldwide. It has also been found in foods such as lettuce and other produce, wheat, cows’ milk, wine, beer, and multivitamins. At high doses, perchlorate
can block iodine from the thyroid gland. Since iodine is needed to make thyroid hormone, thyroid hormone levels might be decreased with even low-level exposure.
Almost everyone in the United States is likely exposed to perchlorate. In one study, higher levels of perchlorate in the urine was associated with lower blood thyroid hormone levels. However, recent studies in pregnant women have shown no link between being exposed to perchlorate and having a change in thyroid hormone levels. Even though research is ongoing and the effects of low-level perchlorate on the thyroid remain unclear, the Environmental Protection Agency has recently decided to limit perchlorate levels in the US drinking water supply.
Thiocyanate and Cigarette Smoke
Thiocyanate [thigh-oh-SIGH-uh-nate] is a chemical that, like perchlorate, can block the thyroid from absorbing iodine. Thiocyanate is found in cigarette smoke and plant foods such as cassava, cabbage, turnips, broccoli, Brussels sprouts, and cauliflower. Large studies testing the effects of cigarette smoking on thyroid function have had varied results. However, it is known that women who smoke during pregnancy are more likely to give birth to babies with low thyroid hormone levels in their blood. Women in the first trimester of pregnancy have lower thyroid hormone levels when they are smokers vs. non-smokers. A recent study showed that cigarette smoking lowers the amount of iodine in breast milk. This may be related to the thiocyanate in The Thyroid and the Environment By Elizabeth N. Pearce, MD, MSc Dr. Pearce is Associate Professor of Medicine at Boston University School of Medicine. She received her medical degree from Harvard Medical School and a Master of Science in Epidemiology from Boston University School of Public Health. Dr. Pearce’s interests include the sufficiency of dietary iodine in the US, thyroid function in pregnancy and lactation, the thyroid effects of environmental perchlorate exposure and other potential endocrine disruptors, and the cardiovascular effects of subclinical thyroid dysfunction. EMPOWER MAGAZINE • Vol . 4, Issue 1 23 cigarette smoke. Diets high in thiocyanate can be part of the reason someone develops goiter (enlarged thyroid) in parts of the world where there is not enough iodine in the diet.
PCBs
In the past, PCBs were used as coolants and lubricants in transformers, capacitors, and other electrical equipment. Starting in the late 1960s there were concerns about the toxicity of PCBs and their ability to persist in the environment. Due to these concerns, production of PCBs was outlawed in the US in 1979. Although levels of PCBs have decreased, PCBs remain widespread in the environment and the food chain because their presence persists for years. The structure of PCBs is similar to that of thyroid hormone, and they are thought to alter the actions of thyroid hormone in body tissues. Babies exposed to PCBs before birth have lower intelligence. This might be because PCBs interfere with the way thyroid hormone helps the brain develop normally.
Bisphenol-A
Bisphenol [BISS-feh-nol]-A (BPA) is used in food containers, baby bottles, and reusable water bottles, and is found in linings of some metal food cans. It may leach from these containers into stored food and drink. Studies in rats have shown that BPA can block thyroid hormone actions, but this has not been clearly shown in humans.
Triclosan
Triclosan [try-KLO-san] is an antibacterial [an-ti-bak-TEERee-ul] agent that is found in soaps, toothpastes, skin care products, plastics, and fabrics. At high doses in rats, triclosan decrease thyroid hormone levels. Lower-level triclosan
exposure has had varying effects on thyroid hormone actions in frogs. In the only human study, brushing teeth with a triclosan-containing toothpaste for two weeks raised blood triclosan levels, but did not alter thyroid function.
PBDEs
PBDEs have been used as flame retardants in plastics, foams, building materials, carpet, and upholstery. PBDEs are slowly released from these products into the environment. PBDEs have been detected in many foods. Exposure may also come from inhaling indoor air and contact with house dust. In animal studies, PBDE exposure causes low thyroid hormone levels. However, results of the few human studies, to date, have not shown consistent effects of PBDEs on the thyroid.
Isoflavones
Isoflavones [eye-so-FLAY-vones] are found naturally in soy products, peas, beans, nuts, grain products, coffee, and tea. Large doses can decrease thyroid hormone. Infants fed soy formula without enough iodine nutrition may develop low thyroid function. Since all infant formulas marketed in the US now contain iodine, this is not currently a problem. Recently 13 out of 14 studies of the effects of soy or isoflavones on thyroid function in healthy adults showed only a small decrease in thyroid hormones or no effects.
Sunscreens
Studies in rats have shown that ingredients in certain sunscreens may alter the body’s ability to process thyroid hormone. These sunscreen ingredients have been found in wastewater treatment plants, are known to build up in fish, and have been found in human milk. Sunscreens, cosmetics, and diet can expose a person to these thyroid hormone-altering ingredients. In one human study, one week of applying sunscreen with these ingredients to the entire body every day did not alter thyroid function.
Summary
Common environmental exposures such as cigarette smoke may affect thyroid function. People may be most vulnerable to these effects in early life, since thyroid hormone is needed for normal brain development. More studies are needed to better understand the risks. 
Dr. Pearce is Associate Professor of Medicine at Boston
University School of Medicine. She received her medical degree from Harvard Medical School and a Master of Science in Epidemiology from Boston University School of Public Health. Dr. Pearce’s interests include the sufficiency of dietary iodine in the US, thyroid function in pregnancy and lactation, the thyroid effects of environmental perchlorate exposure and other potential endocrine disruptors, and the cardiovascular effects of subclinical thyroid dysfunction.
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