No one knows the exact causes
of thyroid cancer. Doctors can seldom explain
why one person gets this disease and another does
not. However, it is clear that thyroid cancer
is not contagious. No one can "catch"
cancer from another person.
Research has shown that people with certain
risk factors are more likely than others to
develop thyroid cancer. A risk factor is anything
that increases a person's chance of developing
The following risk factors are associated with
an increased chance of developing thyroid cancer:
* Radiation. People exposed
to high levels of radiation are much more likely
than others to develop papillary or follicular
One important source of radiation exposure
is treatment with x-rays. Between the 1920s
and the 1950s, doctors used high-dose x-rays
to treat children who had enlarged tonsils,
acne, and other problems affecting the head
and neck. Later, scientists found that some
people who had received this kind of treatment
developed thyroid cancer. (Routine diagnostic
x-rays—such as dental x-rays or chest
x-rays—use very small doses of radiation.
Their benefits nearly always outweigh their
risks. However, repeated exposure could be harmful,
so it is a good idea for people to talk with
their dentist and doctor about the need for
each x-ray and to ask about the use of shields
to protect other parts of the body.)
Another source of radiation is radioactive
fallout. This includes fallout from atomic weapons
testing (such as the testing in the United States
and elsewhere in the world, mainly in the 1950s
and 1960s), nuclear power plant accidents (such
as the Chornobyl [also called Chernobyl] accident
in 1986), and releases from atomic weapons production
plants (such as the Hanford facility in Washington
state in the late 1940s). Such radioactive fallout
contains radioactive iodine (I-131). People
who were exposed to one or more sources of I-131,
especially if they were children at the time
of their exposure, may have an increased risk
for thyroid diseases.
People who are concerned about their exposure
to radiation from medical treatments or radioactive
fallout may wish to ask the Cancer Information
Service at 1-800-4-CANCER about additional sources
* Family history. Medullary
thyroid cancer can be caused by a change, or
alteration, in a gene called RET. The altered
RET gene can be passed from parent to child.
Nearly everyone with the altered RET gene will
develop medullary thyroid cancer. A blood test
can detect an altered RET gene. If the abnormal
gene is found in a person with medullary thyroid
cancer, the doctor may suggest that family members
be tested. For those found to carry the altered
RET gene, the doctor may recommend frequent
lab tests or surgery to remove the thyroid before
cancer develops. When medullary thyroid cancer
runs in a family, the doctor may call this "familial
medullary thyroid cancer" or "multiple
endocrine neoplasia (MEN) syndrome." People
with the MEN syndrome tend to develop certain
other types of cancer.
A small number of people with a family history
of goiter or certain precancerous polyps in
the colon are at risk for developing papillary
* Being female. In the United
States, women are two to three times more likely
than men to develop thyroid cancer.
* Age. Most patients with
thyroid cancer are more than 40 years old. People
with anaplastic thyroid cancer are usually more
than 65 years old.
* Race. In the United States,
white people are more likely than African Americans
to be diagnosed with thyroid cancer.
* Not enough iodine in the diet.
The thyroid needs iodine to make thyroid hormone.
In the United States, iodine is added to salt
to protect people from thyroid problems. Thyroid
cancer seems to be less common in the United
States than in countries where iodine is not
part of the diet.