A new study suggests that people living in areas exposed to high levels of artificial lights at night may be facing higher risks of developing thyroid cancer later in life.

The latest report published online in the American Cancer Society's peer-reviewed journal examines the effect of nighttime lighting. Since rapid industrialization has led to the increased presence of artificial lighting, especially in urban areas. Furthermore, previous epidemiological studies have found a connection between higher levels of satellite-measured nighttime lighting and breast cancer risk.

Investigating Nighttime Lighting and Thyroid Cancer

Since some forms of breast cancer share a common hormone-dependent basis with thyroid cancer, a research team led by Qian Xiao, Ph.D., from the School of Public Health at The University of Texas Health Science Center at Houston, investigated whether nighttime lighting also affected the development of thyroid cancer later in life. Their study involved participants from the NIH-AARP Diet and Health Study, covering American adults aged 50 to 71 years old in the 1995-1996 study duration.

To assess nighttime lighting conditions, researchers pored over satellite imaging data to assess participants' residential locations. They also reviewed state cancer registry databases to identify participants in the study who were diagnosed with thyroid cancer in 2011.

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In the 464,371 participants included in the study, followed for an average period of 12.8 years, researchers found 856 cases of thyroid cancer - 384 of men and 472 women. Comparing the data with the lowest quintile (one of five parts) of nighttime lighting, the study's highest quintile showed 55 percent higher risks of having thyroid cancer.

Furthermore, the study reveals that this association was dominated by thyroid cancer's most common form: papillary thyroid cancer. This was also noted to be more present in women compared to men. However, in women, this association is strongly displayed as a localized form of cancer with almost no signs of spreading to other parts of the body. On the other hand, the connection found also led to more advanced stages and forms of cancer. Researchers also noted how the association appeared unaffected by different tumor sizes, sociodemographic backgrounds, and body mass index (BMI).

Requiring Additional Epidemiologic Studies

In their report, researchers stressed the need for additional studies to confirm the association they found. Once confirmed, this could shed light on the relationship between nighttime lighting and thyroid cancer and the mechanisms that allow for the development of this condition.

One possibility noted in the study is that night lights suppress melatonin, which modulates estrogen activity and has been speculated to have properties that prevent tumor growth. Another possibility is that artificial lighting at night disrupts the internal clock of the human body, or its circadian rhythm, which affects sleep patterns and is a risk factor for different kinds of cancer.

"As an observational study, our study is not designed to establish causality," Xiao said in a statement. "Therefore, we don't know if higher levels of outdoor light at night lead to an elevated risk for thyroid cancer." However, she also adds that the presence of documented evidence regarding the role of nighttime light exposure on the body clock could motivate other researchers to work on the relationship between nighttime lighting and thyroid cancer.

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