Disclosures: The authors report no relevant financial disclosures.
March 01, 2021
3 min read

Exposure to artificial light at night may increase risk for thyroid cancer

Disclosures: The authors report no relevant financial disclosures.
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Living in an area with higher levels of outdoor artificial nighttime light appeared associated with higher risk for developing thyroid cancer, according to an observational study published in Cancer.

“Recent research suggests that light at night, a potent disruptor of circadian rhythms, may be a risk factor for various diseases, including cancer,” Qian Xiao, PhD, MPH, assistant professor in the department of epidemiology, human genetics and environmental sciences at The University of Texas Health Science Center at Houston School of Public Health, told Healio.

Living in an area with higher levels of outdoor artificial nighttime light appeared associated with higher risk for developing thyroid cancer.

“Specifically, a few large studies showed a relationship between higher [light at night (LAN)] measured by satellite images and higher risk [for] postmenopausal breast cancer in the U.S.,” Xiao added. “Thyroid cancer shares some etiologic factors, such as hormonal factors and obesity, with postmenopausal breast cancer and we therefore hypothesized that LAN may also be associated with higher risk [for] thyroid cancer.”

The analysis included 464,371 participants (60.5% men) from the NIH-AARP Diet and Health Study, a large cohort of American adults aged 50 to 71 years at study baseline (1995-1996). Researchers obtained data from the U.S. Defense Meteorological Satellite Program to measure outdoor artificial LAN — which they linked to study participants’ residential addresses to categorize the participants into quintiles of LAN exposure, with the lowest quintile serving as the reference group — and they examined state cancer registry databases to identify thyroid cancer diagnoses through 2011.

Researchers conducted Cox regression analyses to determine the relationship between LAN and thyroid cancer risk, with adjustments made for sociodemographic, lifestyle and other environmental factors.

Median follow-up was 12.8 years.

Overall, 856 cases (men, n = 384; women, n = 472) of thyroid cancer occurred. The majority of these were papillary thyroid cancer (n = 635), followed by follicular (n = 134), medullary (n = 29) and anaplastic (n = 29) types.

Compared with those in the lowest LAN quintile, individuals in the highest quintile of had a 55% increased risk for developing incident thyroid cancer (HR = 1.55; 95% CI, 1.18-2.02).

Overall, women (HR = 1.81; 95% CI, 1.26-2.6) appeared at greater risk than men (HR = 1.29; 95% CI, 0.86-1.94), although the difference between the two did not reach statistical significance.

Among women, researchers found the association between LAN and thyroid cancer appeared stronger for localized cancer (HR = 1.95; 95% CI, 1.27-2.99) than for regional/distant disease (HR = 1.39; 95% CI, 0.61-3.17). Conversely, the association among men appeared stronger for advanced-staged disease (HR = 2.63; 95% CI, 1.24-5.58) than for localized cancers (HR = 1.05; 95% CI, 0.58-1.89).

Qian Xiao, PhD, MPH
Qian Xiao (Source: UT Health)

“Our study adds to a growing body of evidence showing unintended health consequences of high LAN in modern societies,” Xiao told Healio. “However, given that ours is an observational study that is not designed to establish causality, our results should not be interpreted as LAN causes thyroid cancer.”

Also, Xiao and colleagues noted they used one-time LAN measurements at baseline, which may not reflect long-term LAN exposure, and they did not have any information on indoor LAN levels.

“This is a caveat of our study — that we don’t really know what’s going on inside people’s houses at night,” Xiao said. “However, people are exposed to indoor LAN from lights, computers, TV and cellphones, and given the well-established role of LAN in disrupting circadian rhythms, it is generally believed that people should avoid or reduce LAN before bedtime to promote better sleep.

“Future research using personal detectors for light can offer better measurement of LAN,” Xiao added. “Moreover, experimental studies in human populations and animals can help clarify whether there is a causal relationship between LAN and thyroid cancer, and identify the underlying mechanisms that may drive the observed association.”

For more information:

Qian Xiao, PhD, MPH, can be reached at 1200 Pressler St., Rm. RASE603, Houston, TX 77030; email: qian.xiao@uth.tmc.edu.