Functional hyperprolactinemia and relative hyperestrogenism are risk factors of the development concerning fibrocystic changes in the breast.
http://www.ncbi.nlm.nih.gov/pubmed/12375569
Low progesterone activity in all women and low estrogen activity in 65.3% were shown. Absolute high estrogen activity was diagnosed only in 8.7% women with gross breast cysts.
http://www.ncbi.nlm.nih.gov/pubmed/12722409
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Hypothyroidism and low iodine intake may be important aetiological factors in oestrogen dependent tumours of the breast, uterus and ovary. They are preventable risk factors. Iodine supplementation will hopefully lead to a decreased incidence of these cancers in future generations. Oestrogen receptor density may be increased in hypothyroidism as is certain monoamine receptor density. This would amplify the effect of high circulation oestrogen levels in hypothyroidism and may help explain why hypothyroidism and low iodine intake are risk factors for breast, uterine and ovarian cancer.
http://www.ncbi.nlm.nih.gov/pubmed/3067061