About 2 percent of all women suffer from a common thyroid problem called hypothyroidism. The condition occurs when the thyroid gland fails to produce enough thyroid hormone.

More than a century ago, physicians described using thyroid extract to treat breast cancer. Later, studies showed one of the circulating thyroid hormones actually sustains breast cancer cells, while animal studies found mammary gland cells responded to thyroid hormones. Population studies have shown conflicting patterns in the relationship between thyroid hormone and breast cancer.

Researchers from the M. D. Anderson Cancer Center in Houston recently examined the association between the thyroid hormone and breast cancer. The study compared the medical records from 1,136 women with breast cancer and 1,088 healthy women to look at characteristics in women with hypothyroidism and those not affected by the condition.

Study results revealed women with primary hypothyroidism had a 61-percent lower risk of developing invasive breast cancer. In addition, women with breast cancer were 57-percent less likely to have hypothyroidism compared to healthy women. The study also showed women with hypothyroidism were diagnosed with earlier stage breast cancer and had smaller tumors.

The authors write, “The observed association between hypothyroidism and breast cancer may be due to the biologic effect of the thyroid hormone at the cellular level.” They recommend further studies for preventive and therapeutic purposes.