Ria, if you have not already, consider speaking with a board-certified genetic counselor. They are experts in hereditary cancers. There can be a link between thyroid and breast cancer called Cowden Syndrome which can be caused by a PTEN mutation. You can view this article for more information:
http://www.facingourrisk.org/newsletter/2008spring/cowden_syndrome.html
Warm regards,
Sue