Autoimmune hypothyroidism and breast cancer in the elderly

Purpose — To determine whether autoimmune hypothyroidism (AIHT) influences breast cancer (BRCA) incidence or all-cause survival.

Methods — Administrative data were used to identify elderly women living in Ontario, Canada with and without AIHT based on prescriptions for levothyroxine (LT4) (N = 178,186). Women were followed from April 1, 1994–March 31, 2003 for BRCA outcomes.

Results — The incidence of BRCA was similar in LT4 users and a propensity-matched cohort of non-users (adjusted HR = 0.99; 95% CI: 0.92–1.07). All-cause mortality was significantly lower in LT4 users compared to non-users (HR 0.95; 95% CI: 0.93–0.97, P < 0.001), as was all-cause mortality following BRCA diagnosis (HR 0.87; 95% CI: 0.77–0.98, P = 0.02).

Conclusions — LT4 use, as a surrogate for AIHT, is not a risk factor for BRCA in elderly women. The small survival advantage noted among LT4 users who developed BRCA likely represents a ‘healthy user’ effect. Further study is needed to investigate whether autoimmunity in general is related to BRCA development or related outcomes.