Thyroid cancer is on the rise, and it's a concerning trend that has experts scratching their heads. The increase in cases is particularly puzzling because it's happening faster than any other cancer, and it's not just limited to one region or country. So, what's behind this mysterious surge in thyroid cancer diagnoses?
The Thyroid Gland: A Vital Organ
The thyroid gland, a small yet mighty organ located at the base of our neck, plays a crucial role in regulating our body's functions. It releases hormones that control our heart rate, blood pressure, body temperature, and weight. When the cells within this gland start to grow uncontrollably, forming a tumor, it leads to thyroid cancer. These abnormal cells can invade nearby tissues and even spread to other parts of the body.
While most cases of thyroid cancer are treatable, the rapid increase in its incidence is a cause for concern among health professionals. According to data from the Surveillance, Epidemiology, and End Results (SEER) database, a cancer reporting system in the US, the incidence of thyroid cancer in the country more than tripled between 1980 and 2016. This is a significant jump, and it begs the question: why is this happening?
The Rise of Thyroid Cancer: A Mysterious Epidemic
Sanziana Roman, an endocrine surgeon at the University of California, San Francisco (UCSF), puts it bluntly: "Thyroid cancer remains one of the few cancers that has been on the rise over time, despite advances in medicine." But what could be driving this uptick in cases?
Historically, exposure to large amounts of ionizing radiation, especially during childhood, has been linked to thyroid cancer. The aftermath of the Chernobyl nuclear accident in 1986 saw a dramatic increase in thyroid cancer cases among children in Belarus, Ukraine, and Russia. Studies also suggest that about 36% of thyroid cancer cases among Japanese atomic bomb survivors since 1958 could be attributed to childhood radiation exposure.
However, there were no similar nuclear disasters in the US during the '80s or '90s, so experts had to look elsewhere for answers.
Better Diagnosis or Overdiagnosis?
Initially, experts were baffled by the rise in thyroid cancer cases, but an intriguing explanation soon emerged: could it be due to better diagnosis techniques?
In the 1980s, doctors began using thyroid ultrasonography, an imaging technique that uses sound waves to create pictures of the thyroid gland. This allowed physicians to detect very small thyroid cancers that would have otherwise gone unnoticed. Additionally, in the 1990s, medics started performing fine needle aspiration biopsies, where cells are collected from suspicious lumps to determine if they are cancerous.
These advanced diagnostic methods led to an overdiagnosis of thyroid cancer, resulting in many people undergoing unnecessary medical interventions. Cari Kitahara, an epidemiologist at the National Cancer Institute in Maryland, explains: "In the past, physicians would feel the thyroid gland to look for nodules. But with techniques like ultrasonography, doctors could pick up smaller-sized nodules and then biopsy them. This led to increased detection of small-sized papillary thyroid cancers that in the past wouldn't have been felt through palpation."
Other evidence supports the overdiagnosis theory. For instance, while thyroid cancer cases were increasing, the death rate from this cancer remained stable. Additionally, when a national thyroid cancer screening program was introduced in South Korea, the incidence of thyroid cancer skyrocketed, only to decrease again when the program was scaled back.
"Together, these patterns were consistent with overdiagnosis, or the increased detection of disease that would probably never cause symptoms or death if left undetected," Kitahara says.
The Impact of Overdiagnosis
We now know that small papillary thyroid cancers are usually slow-growing and respond well to treatment. They are rarely fatal and have a good prognosis. However, the overdiagnosis of these cancers led to many people undergoing unnecessary medical procedures, including the total removal of the thyroid gland, followed by radioactive iodine treatment to eliminate any remaining cells. These treatments can have side effects, such as vocal cord paralysis and an increased risk of secondary cancers.
To address this issue, clinical practices in the US have changed. Now, radioactive iodine is only used to treat aggressive cancers, and doses are minimized to reduce side effects. Instead of total removal, doctors often partially remove the thyroid gland or take a watchful waiting approach.
As a result, the latest statistics from SEER suggest that thyroid cancer cases have stabilized in the US. For example, in 2010, there were on average 13.9 new cases per 100,000, while in 2022, the last year for which statistics are available, there were 14.1 cases per 100,000.
Beyond Overdiagnosis: Other Factors at Play
Despite the stabilization of thyroid cancer cases in the US, some scientists argue that overdiagnosis alone cannot explain the overall increase in cases worldwide.
Riccardo Vigneri, an emeritus professor of endocrinology at the University of Catania, Italy, points out that if overdiagnosis were the sole explanation, we would expect to see a more significant increase in high-income countries with better diagnostic practices. However, middle-income countries have also experienced a rise in thyroid cancer cases.
Roman adds, "Thyroid cancer rates are increasing even in settings and regions of the world without robust screening. Larger and more advanced tumors are also being diagnosed more frequently. This suggests we are seeing a combination of both detection bias and true increases in disease incidence."
Furthermore, as thyroid cancers are increasingly being diagnosed at earlier stages, and treatment outcomes have improved, one would expect the number of deaths from thyroid cancer to decrease. However, the death rate has remained stable at around 0.5 cases per 100,000 inhabitants, and there are signs that, in some countries, rates are increasing.
For example, a study analyzed more than 69,000 thyroid cancer patients diagnosed between 2000 and 2017 in California. The researchers found that both the number of diagnoses and the mortality rate increased over this period, regardless of tumor size and cancer stage. This suggests that improved diagnosis of extremely small tumors cannot fully explain the increase.
In another study, Kitahara and her team examined the medical records of over 77,000 thyroid cancer patients diagnosed between 1974 and 2013. The results showed that, while most of the increase in cases was driven by small papillary tumors localized to the thyroid gland, there was also an increase in metastatic papillary cancers that had spread to other parts of the body. Although deaths from thyroid cancer are rare, the study revealed that these were increasing at a rate of 1.1% per year.
"This suggested that there could be something else driving the increase in these more aggressive tumors," Kitahara says.
Obesity: A Potential Culprit
One of the main suspects in the rise of thyroid cancer is obesity, which has been on the rise since the 1980s, particularly in the US and other developed countries. Cohort studies suggest a link between excess weight and thyroid cancer risk. People with a high BMI are more than 50% more likely to be diagnosed with thyroid cancer during their lifetime compared to individuals with a healthy BMI.
A high BMI is also associated with aggressive tumor features, such as larger tumor size at diagnosis or mutations that make the cancer more likely to spread. Kitahara's research found that higher BMI was linked to a higher risk of thyroid cancer-related death, suggesting that it's not just detection bias but a true association between obesity and thyroid cancer development and progression.
However, the exact mechanism by which obesity could cause thyroid cancer is still unclear. One known factor is that people with obesity are more likely to have thyroid dysfunction. For instance, individuals with high levels of Thyroid Stimulating Hormone (TSH), which regulates the thyroid gland's function, also tend to have higher BMIs.
"We still don't fully understand the potential underlying mechanisms, as this is still an understudied area of research, but it could be multifactorial," Kitahara says. "Obesity has a lot of physiological effects, so inflammation, insulin resistance, and changes in thyroid function could all play a role in driving the development of thyroid cancer."
Other Potential Factors: Endocrine Disrupting Chemicals and Trace Elements
Other scientists suspect that "endocrine disrupting chemicals" (EDCs) found in common household products and organic pesticides could be responsible for the increase in thyroid cancer cases. These chemicals mimic, block, or interfere with the body's hormones, and examples include perfluorooactanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), which are found in various everyday items.
However, the evidence linking these chemicals to thyroid cancer is mixed. Other studies suggest that trace elements may play a role. Trace elements are chemical elements needed in small amounts by living organisms but are vital for thyroid function.
"We see really high rates of thyroid cancer in island countries," Kitahara says. "There have been hypotheses about trace elements related to volcanic eruptions. So zinc, cadmium, some other chemicals such as vanadium have been shown to be present in these environments, along with high rates of thyroid cancer, but there haven't been a lot of well-designed epidemiologic studies to support a direct link."
Ionizing Radiation: A Possible Contributor
Kitahara believes that another potential explanation for the rise in thyroid cancer cases could be ionizing radiation from diagnostic medical scans. The number of CT and X-ray scans, especially in the United States, has increased significantly since the '80s, and this includes CT scans given to children. These CT scans deliver relatively high doses of radiation to the thyroid gland.
Considering the established relationship between radiation and thyroid cancer, as seen in studies on Japanese atomic bomb survivors, we can estimate the effects of such radiation. For example, one recent study estimated that about 3,500 thyroid cancers a year, moving forward, will be directly attributable to CT scan rates in the United States.
"The young thyroid gland is more vulnerable to the effects of radiation exposure than the thyroid gland in older people," Kitahara says. "So it's possible that the rising use of CT scans could contribute in part to the rising rates of thyroid cancer in the United States and elsewhere."
A Multifactorial Phenomenon
It's likely that a combination of factors is contributing to the increase in thyroid cancer cases. Roman suggests that we are observing a multifactorial phenomenon that includes environmental, metabolic, dietary, and hormonal influences, possibly interacting with underlying genetic susceptibility.
The rise of thyroid cancer is a complex issue, and further research is needed to fully understand the underlying causes. By unraveling these mysteries, we can work towards better prevention and treatment strategies, ensuring better health outcomes for those affected by this disease.
