Serum Levels of Thyroid Hormones in Relation to Breast Cancer Prognosis: A Multicentre Cohort Study

Thyroid function has been suggested to be associated with breast cancer risk, but not much is known about thyroid hormones as a potential prognostic factor for breast cancer. This study investigated the relation between thyroid hormones in breast cancer patients and mortality. A total of 2000 breast cancer patients from the Swedish SCAN-B cohort were followed for 9 years. Blood samples were collected at the time of diagnosis, before surgery, and analysed for free and total T3 and T4. All-cause mortality and recurrent disease were compared between quartiles (Q) using Cox proportional hazard analysis adjusted for prognostic factors, yielding hazard ratios (HR) with 95% confidence intervals (CIs). During follow-up, 310 deaths and 167 recurrences occurred. Free T3 was associated with a lower mortality; the combined group Q2 to Q4 had an adjusted HR of 0.73 (0.58-0.93) as compared to Q1. Moreover, a high ratio of free T3/T4 was associated with a low mortality in Q2, Q3 and Q4, respectively. High levels of free and total T4 were associated with a high risk of all-cause mortality; the crude HR for free T4 Q4 vs. Q1 was 1.53 (1.14-2.07) and the corresponding HR for total T4 was 1.63 (1.20-2.22). These results did not remain in the adjusted analysis. No clear associations were found regarding recurrent disease. We conclude that high levels of free T3, and a high free T3/T4 ratio were associated with a lower risk of all-cause mortality and may be a marker of a favourable prognosis.