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