Nighttime Artificial Lights Linked to Higher Risk of Thyroid Cancer, Study Finds

by DailyHealthPost Editorial

People living in regions with high levels of outdoor artificial light at night may face a higher risk of developing thyroid cancer. The finding comes from a study published early online in CANCER, a peer-reviewed journal of the American Cancer Society.

Link Between Artificial Light and Cancer

Over the past century, artificial lighting at night, especially in cities, have drastically changed due to rapid growth. Previous studies have reported an association between levels of nighttime light and elevated breast cancer risk.

Because some breast cancers may share a common hormone-dependent basis with thyroid cancer, researchers wanted to see if there was also an association between light at night and later development of thyroid cancer.


A team led by Qian Xiao, PhD, of The University of Texas Health Science Center at Houston School of Public Health, looked at data among participants in the NIH-AARP Diet and Health Study, which recruited American adults aged 50 to 71 years in 1995-1996.

The investigators analyzed satellite imagery data to estimate levels of light at night at participants’ residential addresses, and they examined state cancer registry databases to identify thyroid cancer diagnoses through 2011.

Among 464,371 participants who were followed for about 13 years, 856 cases of thyroid cancer were diagnosed (384 in men and 472 in women). They found that those who lived in areas with the highest level of night light pollution had a 55 per cent higher risk of developing thyroid cancer than those in low-light areas.

The association was primarily driven by the most common form of thyroid cancer, called papillary thyroid cancer, and it was stronger in women than in men. 

In women, the association was stronger for localized cancer with no sign of spread to other parts of the body, while in men the association was stronger for more advanced stages of cancer. The association appeared to be similar for different tumor sizes and across participants with different sociodemographic characteristics and body mass index.

The researchers noted that additional epidemiologic studies are needed to confirm their findings. If confirmed, it will be important to understand the mechanisms underlying the relationship between light at night and thyroid cancer. 


The scientists noted that light at night suppresses melatonin, a modulator of estrogen activity that may have important anti-tumor effects. Also, light at night may lead to disruption of the body’s internal clock (or circadian rhythms), which is a risk factor for various types of cancer.

“As an observational study, our study is not designed to establish causality. Therefore, we don’t know if higher levels of outdoor light at night lead to an elevated risk for thyroid cancer.’ 

‘However, given the well-established evidence supporting a role of light exposure at night and circadian disruption, we hope our study will motivate researchers to further examine the relationship between light at night and cancer, and other diseases.” said Dr. Xiao. 

“Recently, there have been efforts in some cities to reduce light pollution, and we believe future studies should evaluate if and to what degree such efforts impact human health.”

The main symptom of thyroid cancer is a lump in the front of the neck. But the disease tends to develop slowly and may not cause any noticeable symptoms at first. However, neck lumps are common and are usually caused by a less serious condition, such as an enlarged thyroid. Only around 1 in every 20 diagnosis is actually cancer. If you notice a swelling or lump at the front of your neck, contact your health practitioner to get it checked out.