Breast cancer in patients with Hashimoto's thyroiditis.

The case records of four groups of women over thirty on the books of a thyroid clinic were used to explore a possible association between breast cancer and Hashimoto's thyroiditis. The index group consisted of 1810 women (10 160 person-years of observation) with Hashimoto's thyroiditis, and the three control groups, matched by sex, age, marital status, and residency, were selected from patients with myxoedema (essential hypothyroidism), benign nodular goitre, or hyperthyroidism. The incidence of breast cancer for the index group, though not for control groups, was significantly higher than that expected from data for the general population, suggesting that patients with Hashimoto's thyroiditis are one high-risk population for breast cancer.