Women with hyperthyroidism appear to have a slight but statistically significant increase (9%) in their risk of developing breast cancer, according to the results of a large, population-based study1 that echoed similar recent findings.2,3 Being hypothyroid had no affect on breast cancer risk. Yet, the women's rate of breast cancer survival was higher.1
"From our preliminary findings, there is a statistically significant increased risk of having breast cancer in people with hyperthyroidism among 100,000 Taiwanese women,"1 Dr. Weng told Endocrine Web. Chien-Hsiang Weng, MD, MPH, residency physician at Dartmouth Family Medicine Residence at Concord Hospital in New Hampshire presented this research at a poster session at ATA on September 22, 2016.
When the authors looked at survival, there was an unexpected finding.
"The 5-year cancer survival was better among those with a thyroid disorder," Dr. Weng told Endocrine Web. The breast cancer mortality rate for those without thyroid disease was 18.3%, while it was 13.5% for women with hypothyroidism, and 11.5% for those with hyperthyroidism.1
Similar findings were echoed by researchers from The Rotterdam Study, which was published online within days of the poster presentation.2 This study confirmed that their data reflected higher fT4 levels were associated with an elevated risk of breast cancer, as well as any solid, non skin cancer, cancers of the gastrointestinal tract and lungs.2 It was also a prospective, population based cohort that included 10,318 participants with a baseline measure of free thyroxine and/or thyrotropin (TSH).2
In April 2016, a team of Dutch researchers were the first to report that hyperthyroidism may be associated with a slightly increased breast cancer risk as compared to the general population.3 The researchers found that hypothyroidism showed a slightly lower albeit not statistically significant risk for breast cancer.3 This study tracked 61,873 women who were hypothyroid and 80,343 women with an initial diagnosis of hyperthyroid disease from 1978 to 2013.3
In vitro studies have shown that high level thyroid hormones have estrogen-like effects and seem to play a role in oncogenic pathways, including that of breast tissue, which gave rise to Dr. Weng's research.
"Thyroid hormones, which plays an important role in growth and cell differentiation, are known to have a direct effect on cancer pathways, specifically increasing proliferation in breast cancer, by affecting gene expression; it has also been hypothesized that thyroid hormone can have a protective effect on some cancers, but there is not conclusive evidence for either of these hypotheses," endocrinologist Minisha Sood, MD, director of inpatient diabetes at Lenox Hill Hospital, New York who reviewed the poster findings for EndocrineWeb.
''The link between thyroid hormone levels and breast cancer is, in fact, plausible," said Dr. Sood; "The Rotterdam Study, for instance, ''lends support to the idea that higher free T4 (free thyroxine) levels increase breast cancer risk."
Dr. Weng suggested that this apparent association with breast cancer may be that women with thyroid disease ”have more regular health care visits such that some hidden health conditions might be more readily discovered and treated."
As for the survival benefit, Dr. Weng suggested that "even after they are diagnosed with breast cancer, these women may have been more compliant with treatment and thus have a better survival." He indicted a notable limitation of the studies was that no information on the types of breast cancer (eg, ER, PR, HER-2 status) was collected.
According to Dr. Weng, ''it is still too early to say if certain preventive guidelines should be revised. But based on these preliminary findings, we would recommend, that patients with hyperthyroidism should be more aware of the increased risk of breast cancer and have regular follow up for breast cancer screening."
Dr. Sood agreed that it was premature to recommend changes in screening guidelines for breast cancer, but she confirmed the value of urging women with hyperthyroidism to become vigilant about getting annual mammograms.
"Many women fear that in treating their hyperthyroidism, they may gain weight or suffer other untoward consequences, but the possible breast cancer connection may be a compelling reason not to ignore their thyroid disease," Dr. Sood said.
Dr. Weng's team evaluated information from the Taiwanese National Health insurance Research Data Base, which is one of the largest administrative health care databases worldwide. The researchers identified women with a primary breast cancer diagnosis in 2006-2011, and no prior cancer history who were then classified with or without hyperthyroidism and hypothyroidism. Thyroid malignancies were not included. The researchers looked at six-year survival from breast cancer.1