A study published July 13, 2009 online in the journal Cancer reported that thyroid cancer rates were increasing for both men and women, and for all sizes of tumors. According to the National Cancer Institute, an estimated 37,200 new cases of thyroid cancer will be diagnosed in 2009.
The study's researchers determined that the incidence of thyroid tumors of all sizes increased, indicating that better detection of smaller thyroid tumors -- through more widespread and aggressive diagnosis using ultrasound and biopsies -- is not the only reason for the increase.
Bruce Davidson, M.D., professor of the Department of Otolaryngology (Head and Neck Surgery) and chair of the Department of Otolaryngology at Georgetown University in Washington, D.C., calls the study findings "quite significant."
Dr. Davidson says, "We've seen thyroid cancer double in the last 10 years, but the general sense in the past was that there was a big pool of undiscovered thyroid cancer, and more intense diagnostics, including greater use of ultrasounds and biopsies, were resulting in better detection. Now, we know that better detection is not the only factor, and we need to investigate other factors that may be causing the increased rate of thyroid cancer."
The study authors have suggested that other risk factors for thyroid cancer -- including environmental, dietary and genetic causes -- need to be explored.
According to Dr. Davidson, one factor that has been connected to the increase in thyroid cancer is obesity, but a definitive link has not been established. Dr. Davidson said, "I don't know if I would attribute the increase in thyroid cancer to obesity."
Radiation is another well-known risk factor for thyroid cancer. The study authors said, "...there is no new evidence to suggest that the exposure of human beings to radiation, a well known environmental risk factor, has increased over time to account for the observed increase in thyroid cancer." According to Dr. Davidson, however, one factor does deserve specific study. He said there was an article out of Europe in past couple of years (an opinion piece) that drew attention to the significant increase in CT scan use over the last 20 years, and the increase in thyroid cancers occurring during the same time. He says, "It's worth looking at to see if there is a connection between low-dose radiation in diagnostics and thyroid cancer. This is particularly true for children, who are more vulnerable than adults. We would want to see whether low-dose radiation exposure in children could trigger thyroid cancer down the line."
The researchers suggested that a number of dietary factors linked to thyroid cancer should be further evaluated, including:
- lower rates of thyroid cancer in those who ate fruits, raw vegetables, and mixed raw vegetables and fruits, as described in one study
- increased rates of follicular cancer in those with a dietary pattern of fish and cooked vegetables, as found in another study
- the correlation between increasing thyroid cancer incidence and increasing dietary iodine intake, as found in another study
One question is whether an increased rate of autoimmune thyroid disease that goes untreated may be contributing to increased rates of thyroid cancer, given that elevated thyroid stimulating hormone (TSH) can increase to risk of thyroid cancer.
According to Dr. Davidson, "Certainly TSH is a growth factor, and we know that growth factors can induce tumors. It makes logical sense that if TSH is chronically elevated, it might increase the risk of thyroid cancer. Whether that can be attributed to the increase in autoimmune disease needs to be studied."
Dr. Davidson also said that various chemicals that have been implicated as possible toxins to the thyroid -- such as perchlorate, dioxins, or environmental estrogens -- are an area ripe for further investigation.
Dr. Davidson says, "In the past, we've heard that the rise in thyroid cancer was 'epidemic.' But now, we have to wonder if maybe it is a true epidemic."
Chen, Amy et. al. "Increasing incidence of differentiated thyroid cancer in the United States, 1988-2005" Cancer, Published Online: 13 Jul 2009 Online.
Telephone Interview with Dr. Bruce Davidson, July 14, 2009.