Overactive Thyroid Linked to Breast Cancer Risk
A team of Danish researchers found that women with thecondition -- called hyperthyroidism -- appeared to face an 11
percent increase in their risk for breast cancer, compared to
women with a normal-functioning thyroid gland.
On the other hand, women with the opposite problem -- a
condition called hypothyroidism, where abnormally low levels of
thyroid hormones are produced -- saw their risk for breast
cancer dip 6 percent below that of women with normal
thyroids.
But at least one breast cancer expert not involved with the
study said she was reluctant to read too much into the study
findings.
The findings stem from a 36-year review that identified
nearly 80,000 Danish women with an overactive thyroid and
more than 61,000 women with an underactive thyroid. All of
the women were cancer-free when they entered the study,
which ran from 1978 to 2013.
Breast cancer incidence was then tracked for roughly every
five to seven years.
Study lead author Dr. Jens Otto Lunde Jorgensen said that
while the review followed patients far longer than previous
efforts, the new findings aren't all that surprising. In fact,
he noted that "a similar association between overactive
thyroid disease and [the] risk of breast cancer in women
has been reported in three out of four previous studies."
At the same time, he stressed that while an overactive
thyroid was found to be associated with breast cancer risk,
one does not necessarily cause the other, adding that "the
possible explanations for this 'association' are many."
"Women with thyroid disease are more likely to see their
doctor and to undergo examinations including mammography,"
Jorgensen said. "Women with overactive thyroid disease also
are predisposed to other diseases such as breast cancer, but
it is not the overactive thyroid by itself. Thyroid hormones may
contribute to development of breast cancer. [And]
treatment of overactive thyroid disease increases the risk of
breast cancer. These are all possibilities, but the bottom line is
that we do not know for sure."
Jorgensen, a clinical professor in the department of
endocrinology and internal medicine at Aarhus University
Hospital in Denmark, and his colleagues report the findings in
the Feb. 11 issue of the European Journal of
Endocrinology.
The researchers pointed out that the thyroid gland is central
to metabolic control. Women are much more likely than men to
have an overactive thyroid, a diagnosis faced by
approximately 51 out of every 100,000 people every year,
researchers said.
Jorgensen added that while thyroid cancer screening is an
increasingly common practice, the new finding "does not
justify increased screening for overactive thyroid disease."
According to the U.S. National Library of Medicine, the
thyroid is a key hormonal gland that plays a major role in the
metabolism, growth and maturation of the human body. It
helps to regulate a variety of functions by constantly
releasing a steady stream of hormones into the bloodstream.
Dr. Courtney Vito, a breast surgeon and an assistant clinical
professor of surgical oncology at the City of Hope
Comprehensive Cancer Center in Duarte, Calif., said it would
be a mistake to make too much out of the study findings.
"First of all, this is a very homogenous group of women," she
said of the Danish participants. "There would never be a
study group like that in the U.S., where there is a lot of
ethnic variation and we are a much more heterogeneous
population. So, these findings might not be generalizable
across other populations."
"There were also plenty of flaws in the study design," Vito
added. For example, she noted that "high thyroid levels were
only shown to have estrogen-like properties in a test tube,
not in actual people. And they [the researchers] didn't
take into account family breast cancer histories, and didn't
comment on how bad anyone's thyroid disease actually was.
"So, I would say that we do know that the endocrine system
is much more complex than our current understanding, and
that breast cancer is clearly a hormonally driven disease that
we clearly need much more [research] to better
understand," she said.
"And this is a study that would make you scratch your head
and think this is interesting, and probably worth a second
look. But there is certainly nothing conclusive here at all."
SOURCES: Jens Otto Lunde Jorgensen, M.D., clinical
professor, department of endocrinology and internal medicine,
Aarhus University Hospital, Aarhus, Denmark; Courtney Vito,
M.D., breast surgeon and assistant clinical professor, surgical
oncology, City of Hope Comprehensive Cancer Center,
Duarte, Calif.; Feb. 11, 2016, European Journal of
Endocrinology
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