Monday, May 4th, 2009
Although experts agree it is difficult to know precisely what causes thyroid cancer, they do say there are potential risk factors to consider. They are risk factors because they don’t cause thyroid cancer 100 percent of the time, but they increase a person’s chances of developing thyroid cancer. Radiation exposure is the only risk factor directly linked to cases of thyroid cancer.
Experts say about 95 percent of papillary and follicular thyroid cancers occur by chance and about 5 percent occur in families with a history of the disease. Papillary cancer grows in small finger-like shapes and often appears as a solid mass or cyst on the thyroid. Follicular cancer, like papillary cancer is slow growing, however, it is more common in countries where people don't get enough iodine in their diet, according to the American Cancer Society.
Researchers investigating the gene defects of papillary cancer have detected several mutations that lead to the cancer.
"However, even here the question would be is it environmental factors that lead to the mutations, or is it a person's particular gene makeup that makes them more prone to the cancer, or both," said Dr. Thomas Moraghan, a North Dakota endocrinologist and former assistant professor of medicine at Mayo Clinic.
On Feb. 6, Decode Genetics, an Icelandic biopharmaceutical company, published a new study about the role genes play in the development of thyroid cancer. Researchers studied the genetic makeup of 40,000 patients from Iceland, the United States and Spain and found two gene variants “contribute to an estimated 57 percent of all cases” of thyroid cancer.
Screening for the gene variants may help identify at-risk patients, said Kari Stefansson, the study’s senior author, according to the company’s Web site. However, some argue universal testing for a rare disease such as thyroid cancer is unwarranted.
Genetic testing is already used for the diagnosis of familial medullary cancer and the type of medullary cancer called MEN 2. Children who have a RET gene mutation , have a 50 percent chance of inheriting the gene defect from a parent. Doctors can remove the thyroids of children who possess the gene mutation before the cancer develops.
Benign thyroid disease
Some experts debate whether individuals presenting benign thyroid disease should also be considered at-risk for developing thyroid cancer. Unlike most thyroid cancers, benign thyroid disease is hereditary.
Studies have shown some thyroid nodules, which are growths or lumps on the thyroid that may or may not be cancerous, can be detected, with imaging techniques, in 40 to 50 percent of the population by the time they reach age 50, said Dr. Robert Smallridge, the chair of endocrinology at Mayo Clinic Jacksonville.
“Fortunately, about 95 percent of all nodules are benign and need to be evaluated before jumping to the conclusion of cancer,” Smallridge said.
In addition to thyroid nodule abnormalities, Smallridge said about 10 percent of women worldwide have chronic thyroiditis – an autoimmune thyroid disease commonly known as Hashimoto’s. Symptoms of the disease often are similar to hypothyroidism in that the person may experience fatigue, mild weight gain and have difficulty concentrating.
And about 5 to 10 percent of women more than 45 years old are hypothyroid, which means their thyroid isn’t producing enough of the hormones on its own, according to Dr. Kenneth Burman, the chief of Endocrinology at the Washington Hospital Center.
All of these thyroid abnormalities mean that about 20 to 25 million Americans are taking a supplement designed to regulate thyroid hormones whether the thyroid has been removed.
Some experts suggest the increase in thyroid cancer could be partly due to pediatricians’ increased use of CT-scans.
Dr. Jennifer Sipos is an Ohio State University endocrinologist who warned experts not to dismiss the disease’s increasing prevalence at the Thyroid Cancer Survivors’ annual conference last September.
Although she said doctors are finding more of the thyroid cancers that would have otherwise gone unnoticed and not caused problems for the patient, she believes other factors could be playing a role.
“We’re using a heck of a lot more radiation for imaging people, so the exact thing that is finding a lot more thyroid nodules, i.e. CAT scans, could be causing more of them to be cancerous,” Sipos said.
Though individuals are exposed to a higher dose of radiation when undergoing a CT-scan than a diagnostic X-ray, no one has shown that CT-scans increase the risk of thyroid cancer, according to Dr. Elaine Ron, a senior investigator at the National Cancer Institute.
Because thyroid cancer is a rare disease, Ron said you would need a study of about a million people to learn the effects of very, low-dose radiation on the thyroid. A study that large has yet to be attempted.
Scientists say certain chemicals disrupt thyroid function, but experts debate whether that disruption leads to cancer.
In 1998 and 2002, the Environmental Protection Agency conducted studies of perchlorate – a salt used in the production of rocket fuel and fireworks – and its ability to inhibit iodine uptake in the thyroid, which leads to hypothyroidism.
Experts disagree on whether such pollutants are found in high enough concentrations to increase a person’s risk for thyroid cancer.
Although Dr. Kenneth Burman, the chief of Endocrinology at Washington Hospital Center, said there’s no official statement on most of these issues, he said perchlorate does have an effect on thyroid tissue. However, he cited studies conducted by Boston University’s Dr. Lewis Braverman , which concluded the amount of perchlorate in water contamination is not enough to affect thyroid function and probably not a factor in causing the increased rate of thyroid cancer.
“I’d say it’s not completely known, but so far it doesn’t look like it is in high enough concentrations,” Burman said.
Dr. R. Thomas Zoeller, the chair of biology at the University of Massachusetts and a reproductive endocrinologist, has a different take on those studies.
“Out of all the studies on perchlorate there’s only a couple that have not been funded by industry,” Zoeller said. “I think that’s a problem because the way they wind up doing a study is pretty much designed to tell you that there’s nothing there.”
He said the concentrations of perchlorate contamination, as determined by a 2006 Centers for Disease Control study , are high enough to affect thyroid function, especially in pregnant women and infants. Thyroid hormones also regulate childhood growth and development, so Zoeller said he believes an inverse relationship exists between an infant’s exposure to perchlorate and his or her cognitive function.
Even so, he said, “I have a hard time turning perchlorate into a possible thyroid cancer causative agent.”
The reason: Zoeller said researchers and clinicians disagree about whether an increase in thyroid-stimulating hormone leads to an increased risk of thyroid cancer. As its name suggests, thyroid-stimulating hormone (TSH) regulates the thyroid’s functioning and growth. He said an increase in TSH does not on its own increase a person’s risk of thyroid cancer.
In their recent study of the rising cancer incidence, Ron and her co-authors noted a study by Copenhagen researchers , who documented the effect of chemicals such as polychlorinated biphenyls and dioxins on the thyroid.
Ron and co-authors noted the correlation between chemicals and increased TSH and the resulting potential “in an increased opportunity for mutations and the development of cancer.”
Diet and obesity
Because the thyroid needs iodine to produce thyroid hormones, researchers have found iodine deficiency adversely affects thyroid health and human growth and development.
“I tell my patients to eat a normal, healthy American diet and you’re going to get plenty of salt, plenty of iodine,” Sipos said.
She said some have speculated about processed foods and whether anything else in our diet might be causing the increase in thyroid cancer. “It is a world-wide phenomenon, so that’s going to have to be a little bit more of a ubiquitous substance than just iodine because iodine intake tends to be regional,” Sipos said.
As with many cancers, scientists are also examining whether increased body mass index plays a role in an individual’s risk of developing thyroid cancer.