Women treated for hypothyroidism were less likely to develop primary breast carcinoma—and more likely to have more indolent disease when they did develop cancer—than women with normal thyroid function in a retrospective, case-control study.
The findings “strengthen the possibility of a significant role of thyroid hormones on breast cancer biology” and may indicate areas of intervention “for targeted preventive and therapeutic purposes,” said Massimo Cristofanilli, M.D., and his associates at the University of Texas M.D. Anderson Cancer Center, Houston.
The investigators compared the medical records from 1,136 women with primary breast carcinoma with those of 1,088 healthy women who visited their breast cancer-screening clinic.
Approximately 240 women reported having symptomatic primary hypothyroidism and thyroid supplementation before they were diagnosed with breast cancer or before their screening visit.
The prevalence of reported hypothyroidism was significantly greater in the control group, compared with the breast cancer group (15% vs. 7%).
Women with primary hypothyroidism had a 61% lower risk of developing invasive breast cancer, and women with breast cancer were 57% less likely to have hypothyroidism, compared with healthy women, the investigators reported (Cancer 2005;103:1122–8).
After adjustment for confounders of breast cancer risk such as family history of breast carcinoma and history of pregnancy, the association between breast cancer and hypothyroidism “essentially was unchanged and [treated hypothyroidism] remained a strong protective factor against a diagnosis of invasive breast carcinoma,” they said. The association was similar in all ethnic groups.
Among women with invasive breast cancer, patients with hypothyroidism were older at the time of diagnosis (59 vs. 51 years) and were more likely to be postmenopausal (82% vs. 64%) than patients with normal thyroid function.
Among white women specifically, those with invasive cancer who had been treated for hypothyroidism were more likely to be diagnosed with early-stage disease (95% vs. 86%) and disease without pathologic lymph node involvement (64% vs. 56%).
They also had smaller pathologic tumor size than women with invasive cancer but normal thyroid function.
The investigators restricted part of their analysis to white women to have a more homogenous population and to eliminate the influence of ethnicity.
A possible association between thyroid disease and breast carcinoma has been debated for decades and remains controversial, the investigators say. Epidemiologic studies have had conflicting results, and large prospective studies have failed to clearly demonstrate a correlation.