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Air pollution may alter the baby’s thyroid function during pregnancy

Clinical Thyroidology for the Public

Thyroid hormones are critical for the growth and development of babies during pregnancy. During the first trimester of pregnancy, the mother supplies thyroid hormones. However, starting in the second trimester, the developing baby is able to produce his/her own thyroid hormone. T4 is the major form of thyroid hormone released by the thyroid gland and T3 is the most active form of thyroid hormone. Both are controlled by thyroid stimulating hormone (TSH). Chemicals that disrupt endocrine glands (endocrine disruptors) are being studied. Organic compounds, chemicals such as cadmium and cigarette smoking may affect thyroid regulation. Air pollution is a mixture of several particles, including metals, nitrate and organic materials, which have been shown to affect endocrine glands. Exposure to air pollution has also been associated with an increased risk for low birth weight and preterm birth. However, the role of air pollution on the baby’s thyroid during pregnancy is not known. This study was done to determine whether exposure to air pollution during late pregnancy affects the mother’s and/or the baby’s thyroid function and birth weight.

Janssen BG et al Fetal thyroid function, birth weight, and in utero exposure to fine particle air pollution: a birth cohort study. Environ Health Perspect. September 13, 2016

The authors studied 640 mother–child pairs from the East Limburg Hospital in Genk, Belgium, between February 2010 and June 2014. Mother-children pairs were part of the ENVIRONAGE birth cohort study. They obtained data from small particles in the air, with a diameter equal or less to 2.5 μm, based on the mother’s home address using a system that permits correlation with space and time from satellite images. Using this system, they were able to obtain daily air small particle values using data from the Belgian network that studies air-quality. The outcomes were obtained for the third trimester of pregnancy and the average exposure value was used after correction for environmental factors such as temperature and humidity. Data on subject characteristics was obtained through questionnaires. They also reviewed other factors associated with pregnancies, by reviewing the medical files of the hospital. Umbilical cord and blood samples from the mother were collected at the time of delivery or up to 1 day after delivery. They had data for 499 newborns and 431 mothers.

The range of the small particles in the air was divided into 4 ranges. For each higher range, cord-blood TSH concentrations were 11.6% lower; cord-blood FT4 concentrations were 3.7% lower and cord-blood FT3 concentrations were 6.4% higher. The effect on the mother’s laboratory tests did not reach significance. Only free T4 levels were associated with birth weight. An 11% decrease in cord blood free T4 was associated with a decrease of 56 grams in birth weight, after adjusting for other variables.

This study shows for the first time that exposure to air pollution decreases cord Free T4 and increases cord Free T3 level. Lower cord Free T4 levels were also associated with lower birth weight. This study is important because, although the data is limited, it highlights the need to do more research in environmental health and air pollution.

— Liuska Pesce, MD

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Thyroid Disease and Pregnancy: thyroid-disease-pregnancy/

Thyroid Function Tests: thyroid-function-tests/


Thyroxine (T4): the major hormone produced by the thyroid gland. T4 gets converted to the active hormone T3 in various tissues in the body.

Triiodothyronine (T3): the active thyroid hormone, usually produced from thyroxine.

TSH: thyroid stimulating hormone — produced by the pituitary gland that regulates thyroid function; also the best screening test to determine if the thyroid is functioning normally.

Endocrine disruptors: chemical pollutants in the environment that can affect the action of endocrine glands. Examples include bisphenol A (BPA), polychlorinated biphenols (PCBs), perfluoroalkyl substances (PFAs) and organochlorines (OCs).