High level of estrogen may increase the number of cells in breast cancer. Prior laboratory experiments have shown that high levels of thyroid hormone may have the same effect.
However, the observational and population-based studies have not support this idea consistently. Three previous studies showed higher risk of breast cancer for women who have had hyperthyroidism and one showed higher risk for women with hypothyroidism and high TPO antibody levels; while a review from 2012 did not show any relationship between hypothyroidism and breast cancer.
This study was done to assess any possible association between hyperthyroidism, hypothyroidism and breast cancer risk.
THE FULL ARTICLE TITLE:
Søgaard M et al. Hypothyroidism and hyperthyroidism and breast cancer risk: a nationwide cohort study. Eur J Endocrinol 2016;174:409-14.
SUMMARY OF THE STUDY
This study was performed in Denmark, using the information from Danish National Patient Registry (containing records of hospitalizations since 1977, clinic and emergency room visits since 1995) and Danish Cancer Registry (containing breast cancer records; information about estrogen receptor status of the breast cancers was available since 1997). 4,177,429 women lived in Denmark between 1978 to 2013. Women with hyperthyroidism and hypothyroidism were identified by diagnosis codes used by doctors.