The simple answer is, maybe. At present, the coincidence of both breast cancer and thyroid disorders is still a subject of extensive debate.
Some researchers suggest that there is definite evidence that thyroid abnormalities may influence the progression of breast cancer. But other researchers strongly disagree, and have found contradictory evidence.
What is clear is that more research is needed into the connections between thyroid disease and breast cancer. this becomes even more important, given the recent findings that indicate that thyroid disease is far more prevalent that originally thought, and there are an estimated 13 million Americans unaware of and undiagnosed for their current thyroid conditions.
To aid in your exploration of this issue, here are some summaries and links to key research looking at the relationships between thyroid disease and breast cancer.
Women With Thyroid Cancer at Increased Risk for Breast Cancer
A new study finds that treatment for thyroid cancer in younger women increases their risk for breast cancer five to twenty years later.
"Thyroid hormone action"
The Cancer Journal - Volume 6, Number 5 (September-October 1993)
This research suggests that thyroid hormone has a direct and crucial role in the development of breast cancer. "Thyroid hormone is a very powerful co-factor of experimental carcinogenesis. The role of thyroid hormone in human cancers
has yet to be clearly elucidated."
"The thyroid and breast cancer: a significant association?"
Ann Med 1997 Jun;29(3):189-91
These researchers found that there's no clear evidence of a causal relationship - in that one disease causes the other - but there does appear to be an association between breast cancer and hypothyroidism, in particular. There's also some evidence that dietary iodine may affect breast cancer risk, and there's an increased likelihood of having breast cancer if one has an enlarged thyroid, known as a goiter.
"Thyroid disorders and breast cancer"
Eur J Cancer Prev 1996 Dec;5(6):504-6
The prevalence of hyperthyroidism and hypothyroidism in patients with breast cancer versus those without breast cancer is very similar. Non-toxic goiter (an enlarged thyroid) was more than twice as common in the breast cancer patients; 45.5% of breast cancer patients had thyroid enlargement compared with only 10.5% of controls. And, antithyroid peroxidase autoantibodies were twice as common in breast cancer patients than in controls.
Breast cancer in hypothyroid women using thyroid supplements.
JAMA 1984 Feb 3;251(5):616-9
This study found that use of thyroid supplements does not increase the risk of developing breast cancer.
"Benign thyroid diseases and the risk of death from breast cancer."
This study found that patients with Hashimoto's thyroiditis had the lowest percent of deaths due to breast cancer, while those with nontoxic nodular goiter had the highest.
"Relationship between breast cancer and thyroid disease: relevance of
autoimmune thyroid disorders in breast malignancy."
J Clin Endocrinol Metab, 1996 Mar;81(3):990-4
This research found that the overall prevalence of thyroid disease was 46% in the breast cancer patients studied, versus 14% in the controls studied. The prevalence of nontoxic goiter was 27.4% in breast cancer patients, versus 11% in controls. Hashimoto's thyroiditis was found in 13.7% of breast cancer patients, and in 2% of the controls. The prevalence of thyroperoxidase (TPO) antibody was higher in breast cancer patients than in controls (23.5% vs. 8%). Overall, the study found that the prevalence of thyroid disorders is increased in patients with breast cancer, and thyroid
autoimmune disorders, especially Hashimoto's thyroiditis, account to a large extent for the increased prevalence of thyroid disease in patients with breast cancer.
"Thyroid function after postoperative radiation therapy in patients with
breast cancer. "
Acta Radiol Oncol 1986 May-Jun;25(3):167-70
Post-operative radiation therapy for breast cancer can damage the thyroid, so shielding of the thyroid is recommended during irradiation, as well as long-term follow-up of thyroid function with repeated thyroid testing.