A surprising finding was that a total of fifty-three percent of the patients had thyroid cancer -- many more than the 28% who went into the surgery with a thyroid cancer diagnosis. Even when you exclude the patients who already knew they had thyroid cancer going into the surgery, the prevalence of thyroid cancer in the Hashimoto's patients as still 35.6%. According to the researchers, it is unclear why there is a link between Hashimoto's thyroiditis and cancer -- it may have to do with an immune response. Papillary thyroid cancer in Hashimoto's patients does, however, have a more favorable prognosis than in patients without Hashimoto's.
Shih, Ming-Lang et. al. "Thyroidectomy for Hashimoto's Thyroiditis: Complications and Associated Cancers," Thyroid. July 1, 2008, 18(7): 729-734. Full text PDF
NOTE: Free access to the full July 2008 issue of Thyroid is available online.