https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5327366/
Abstract - full free paper at link - one important factor of several I suspect
"Changes in Dietary Iodine Explains Increasing Incidence of Breast Cancer with Distant Involvement in Young Women"
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In conclusion, dietary iodine insufficiency represents a plausible explanation for the increasing incidence of breast cancer in young women with distant metastasis. In view of the established reduction in iodine levels in US women of childbearing age since the mid 70s, this group would be most vulnerable to increased breast cancer risk. The increased sensitivity of breast tissue to estradiol induced proliferative changes in the setting of dietary iodine insufficiency, provides a plausible mechanistic explanation for the increasing incidence of breast cancer with distant involvement in this age group. Based on the importance of iodine in thyroid and breast health, fetal brain development, as well as deficits in nutritional trends among younger women,
iodine testing and management may be considered as a potentially important aspect for clinical practice."
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