Benign thyroid diseases and the risk of death from breast cancer.

A survey of the causes of death among women with benign thyroid disease was conducted to assess the risk of breast cancer mortality in thyroid patients. The study population was all women diagnosed with one of several types of thyroid disease at the Massachusetts General Hospital Thyroid Clinic from 1925 to 1974. A population-based comparison group was matched to the Clinic patients for age and socioeconomic status, resulting in a total of 9,520 matched pairs. A search of the Massachusetts mortality records located death certificates for 10.9% (1,039) of the Thyroid Clinic patients and 10.5% (995) of the comparison women. Cancer was the cause of death in 21% (218) of the Thyroid Clinic patients and 24% (239) of the comparison women. Fewer patients than comparison women died from cancers of the digestive organs (30.2 vs. 45.5%), but more patients died from thyroid cancer (3.7 vs. 0%), lymphatic and hematopoietic cancers (11.5 vs. 4.2%), and cancers of other sites (8.3 vs. 3.8%). Breast cancer deaths accounted for 21.6% of cancer deaths in the patients and 22.2% of cancer deaths in the comparison women. When specific thyroid diagnoses were considered, the percent of all deaths due to breast cancer ranged from 1.9 to 5.6%, compared to 5.3% in the comparison women. Of the diagnostic groups studied, patients with Hashimoto's thyroiditis had the lowest percent of deaths due to breast cancer, while those with nontoxic nodular goiter had the highest.