Triiodothyronine levels in relation to mortality from breast cancer and all causes: a population-based prospective cohort study.

OBJECTIVE: The potential association between thyroid hormones and breast cancer has been investigated in a large number of studies without conclusive results. This study investigated triiodothyronine (T3) levels in relation to breast cancer mortality in a population with no breast cancer patients at baseline. An additional aim was to study T3 levels in relation to mortality from other cancers and all-cause mortality. DESIGN AND METHODS: This was a population-based prospective cohort study including 2185 women in whom T3 levels were measured as part of a preventive health project, i.e. before diagnosis in women who later developed breast cancer. Mean follow-up was 24.1 years and record-linkage to The Swedish Cause-of-Death registry identified 471 women who died: 26 out of breast cancer and 182 from other cancers. Mortality was assessed using a Cox's analysis, yielding hazard ratios (HRs), with 95% confidence intervals. Analyses of T3 as a continuous variable were repeated for pre- and peri/postmenopausal women separately.
RESULTS: T3 levels were positively associated with the risk of breast cancer-specific death in the age-adjusted analysis: HR for T3 as a continuous variable was 2.80 (1.26-6.25). However, the crude analysis did not reach statistical significance. Breast cancer mortality was even higher in postmenopausal women: 3.73 (1.69-8.22), but stratified analyses included few events. There were no statistically significant associations between T3 levels and deaths from other cancers, age-adjusted HR: 1.09 (0.72-1.65) or all-cause mortality (1.25:0.97-1.60).
CONCLUSIONS: This study, the first of its kind on prospectively measured T3 levels, indicates that T3 levels are positively associated with breast cancer-specific mortality and that this is not related to a general effect on all-cause mortality.