Autoimmune hypothyroidism and breast cancer in the elderly.
OBJECTIVE
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.
Sandhu
Monty K
MK
Department of Medicine, University of Toronto, Toronto, Canada.
Brezden-Masley
Christine
C
Lipscombe
Lorraine L
LL
Zagorski
Branden
B
Booth
Gillian L
GL
D000368:Aged; D001327:Autoimmune Diseases / Q000150:complications*; D001943:Breast Neoplasms / Q000188:drug therapy / Q000453:epidemiology / Q000209:etiology*; D002170:Canada / Q000453:epidemiology; D015331:Cohort Studies; D005260:Female; D019398:Genes, BRCA1; D006801:Humans; D007037:Hypothyroidism / Q000150:complications*; D015994:Incidence; D008207:Lymphatic Metastasis; D011379:Prognosis; D012189:Retrospective Studies; D012307:Risk Factors; D013974:Thyroxine / Q000008:administration & dosage; D016896:Treatment Outcome
D013974:Thyroxine