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