I was told that people with thyroid cancer have an increased risk of developing bc- and vice versa, and having had radiation treatment increases the risk factor a bit more. The research has yet to have a definitive reason for this, it just may another wrinkle caused by an immune system that just doesn't recognize cancer cells as something to be destroyed.
I will probably be hearing about that again at an upcoming appt, as I have developed nodules on my thyroid, (can be felt by physician) that were not felt prior to bc. Imaging shows two nodules, but they were under the size where a biopsy would be recommended, so now that a year has passed, she may want another image done to assess. It may have nothing to do with bc, as a lot of people get nodules as they age. Wishing you all the best as you figure out what is going on with your thyroid, too! Hugs!