Thyroid hormones control many activities in the body, including how fast we burn calories. They also control how fast the heart beats and how children grow and develop. When the body has too little or too much of these hormones, it may feel restless or tired, or may lose or gain weight. Some chemicals in the environment may change the way thyroid hormones normally work in the body. Chemicals called “thyroid inhibitors” can reduce or prevent the body from making thyroid hormones. This may lead to health problems. A chemical sometimes found in drinking water, called perchlorate, is an example of one of these chemicals.

We examine how pollution or chemicals in the air or water may have an effect on thyroid hormones. We study how certain chemicals may affect thyroid hormones in pregnant women and girls. We also look at how a mother’s thyroid hormone levels may affect her baby's weight at birth, as well as premature birth and autism.

Thyroid hormone levels during pregnancy are very important. They can affect the mother’s and baby's health. The chemical perchlorate causes the body to make less thyroid hormone. EHIB and collaborating scientists studied pregnant women in San Diego County and measured perchlorate levels in their urine and their thyroid hormone levels. We found lower levels one thyroid hormone (thyroxine or “T4”), in pregnant women with higher perchlorate levels (1). Baby boys of mothers with higher perchlorate levels were born earlier and also larger (2).

Thyroid hormones play a key role in how a baby’s brain grows and develops during pregnancy and early life. Autism is a disorder of the brain. Routine testing in newborns includes checking for a condition called hypothyroidism. This condition causes the body to not make enough thyroid hormones. Severe hypothyroidism in infants may lead to slower growth and delayed development. We worked with research partners to examine the relation of thyroid hormone levels and autism in three different studies (3)(4)(5). The study found different things. Two studies suggested that children with autism had very low levels of one thyroid hormone (T4) as newborns (3)(4). In another study (5), we found that pregnant women who had low levels of another thyroid hormone (thyroid stimulating hormone or TSH) were more likely than other mothers to have autism develop in their children.

Development and puberty are controlled by many hormones. Learning more about how thyroid hormones affect growth can help scientists understand the effects of certain chemical exposures on thyroid hormones. We studied thyroid levels in young girls in California. We found that girls with higher thyroid hormones reached puberty earlier (6). We also participated in a larger research study with scientists in New York and Ohio to examine thyroid inhibitors in girls in three States. This larger study found that higher levels of thyroid inhibitors (perchlorate, thiocyanate, and nitrate) in girls’ urine was associated with slower growth (7). This research was part of the Breast Cancer and the Environment Project (BCERP), a study jointly funded by the National Institute of Environmental Health Sciences and the National Cancer Institute.

These studies by EHIB and collaborating scientists help shed light on how exposures in our environment may affect health during pregnancy, birth and developmental outcomes by altering thyroid hormone levels. See below for links to the research summaries.