Thyroid Cancer Rates Triple, and Scientists Look for Cause

Thyroid cancer rates are rising faster than any other cancer in the United States, a new study found: Between 1975 and 2013, the number of thyroid cancer cases diagnosed yearly more than tripled.

The numbers have prompted many epidemiologists to caution in recent years that the increase in cases is really just a matter of doctors catching more cases. This includes cases that are slow-growing and that would be unlikely to cause symptoms that affect a person's life. Doctors refer to the diagnosis of cases like this as the "overdiagnosis" of a condition.

But in the new analysis, scientists argued that the alarming rise isn't just due to improvements in detecting thyroid cancer. [10 Do's and Don'ts to Reduce Your Risk of Cancer]

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"While overdiagnosis may be an important component to this observed epidemic, it clearly does not explain the whole story," said Dr. Julie Sosa, one of the authors of the new study and the chief of endocrine surgery at Duke University in North Carolina.

Sosa, along with epidemiologists at the National Cancer Institute, acknowledged in their new study that better tools — from diagnostic ultrasound to fine-needle biopsies — have improved doctors' ability to detect thyroid cancers. But the study, published today (March 31) in the Journal of the American Medical Association (JAMA), said that thyroid cancer is a real, growing threat, as shown by the increasing cases of a type of thyroid cancer called advanced stage papillary thyroid cancer, along with a steady rise in deaths from the disease.

In the study, the team analyzed more than 77,000 cases of thyroid cancer documented in a National Cancer Institute (NCI) database between 1974 and 2013. Along with the tripling in cases over that period, the researchers noted that between 1994 and 2013, cases of advanced forms of thyroid cancer rose by about 3 percent each year, and deaths from the disease rose by about 1 percent each year.

In the period from 1974 to 1977, there were 4.6 cases of thyroid cancer per 100,000 people diagnosed yearly in the U.S. That number reached 14.4 cases per 100,000 people yearly in the period from 2010 to 2013.

Currently, more than 60,000 Americans are diagnosed each year with a form of thyroid cancer, according to the NCI. About 75 percent of these patients are women, and 82 percent are white, the researchers found.

That deaths from thyroid cancer are increasing, despite it being among the most treatable and least lethal forms of cancer, is worth noting, Sosa said.

"Overall, we are starting to win the war against cancer, but this is one of the few cancers where we're actually losing ground," Sosa told Live Science. "That means we have to now understand what the possible explanations are for the increased incidence of thyroid cancers."

What's causing the increase in cases?

In the study, the researchers offered several possible explanation for the increase in thyroid cancer cases. Rising obesity rates in the United States could be one factor, the authors said.

As cases of thyroid cancer rose in the United States, so did obesity. The number of obese U.S. adults tripled between 1960 and 2012, with the fastest rate of increase occurring between 1980 and 2010, according to the study. Previous research, including a 2012 study in the journal JAMA Surgery, has shown that as patients' body mass indexes (BMIs)increase, their risk of developing more aggressive forms of thyroid cancer also increases.

Another possible factor mentioned in the study may come as a surprise: a decrease in smoking.

Although smoking has been shown to increase risk for a host of other health problems — including cardiovascular disease, stroke, lung cancer and other cancers — it oddly has been linked to a decreased risk of developing thyroid cancer, the study said.

The researchers found that smoking was associated with a 30 to 40 percent lower chance of developing thyroid cancer.

"No one is suggesting that people start up smoking to avoid thyroid cancer," cautioned senior study author Cari Kitahara, an epidemiologist at the National Cancer Institute. "It's just an interesting association that we see in our data, and it provides some clues to what factors are involved in thyroid cancer development." [7 Cancers You Can Ward Off With Exercise]

Yet another factor could also play a role: exposure to a group of chemicals and pollutants known as polybrominated diphenyl ethers (PBDEs). These compounds are found in the environment, as well as in food and people's homes. PBDEs include flammable-resistant chemicals in clothing and furniture, certain pesticides, and some chemicals found in foods and plastics.

Studies have shown that PBDEs can interfere with the thyroid gland and other glands in the body's endocrine system. (This is the collection of glands that produce and regulate hormones.) 

"Certain environmental pollutants could be a factor since we're receiving so much exposure to chemicals that could be endocrine-disruptive," Kitahara said.

The problem is that PBDEs are very widespread, so proving a link between exposure to the chemicals and an increase in thyroid cancer is a real challenge, Kitahara said.

As Kitahara told Live Science, "there is just very limited data."

Originally published on Live Science.