title: Hypothyroidism and hyperthyroidism and breast cancer risk: a nationwide cohort study.
abstract: OBJECTIVE
  The association between thyroid disease and breast cancer risk remains unclear. We, therefore examined the association between hypothyroidism, hyperthyroidism and breast cancer risk.
  
  
  METHODS
  This was a population-based cohort study.
  
  
  METHODS
  Using nationwide registries, we identified all women in Denmark with a first-time hospital diagnosis of hypothyroidism or hyperthyroidism, 1978-2013. We estimated the excess risk of breast cancer among patients with hypothyroidism or hyperthyroidism compared with the expected risk in the general population, using standardized incidence ratios (SIRs) as a measure of risk ratio. Breast cancer diagnoses in the first 12 months following diagnosis of thyroid disease were excluded from the calculations to avoid diagnostic work-up bias.
  
  
  RESULTS
  We included 61, 873 women diagnosed with hypothyroidism and 80, 343 women diagnosed with hyperthyroidism. Median follow-up time was 4.9 years (interquartile range (IQR): 1.8-9.5 years) for hypothyroidism and 7.4 years (IQR: 3.1-13.5 years) for hyperthyroidism. Hyperthyroidism was associated with a slightly increased breast cancer risk compared with the general population (SIR: 1.11, 95% CI: 1.07-1.16), which persisted beyond 5 years of follow-up (SIR: 1.13, 95% CI: 1.08-1.19). In comparison, hypothyroidism was associated with a slightly lower risk of breast cancer (SIR: 0.94, 95% CI: 0.88-1.00). Stratification by cancer stage at diagnosis, estrogen receptor status, age, comorbidity, history of alcohol-related disease and clinical diagnoses of obesity produced little change in cancer risk.
  
  
  CONCLUSIONS
  We found an increased risk of breast cancer in women with hyperthyroidism and a slightly decreased risk in women with hypothyroidism indicating an association between thyroid function level and breast cancer risk.
authors: Departments of Clinical EpidemiologyEndocrinology and Internal Medicine (MEA)Aarhus University Hospital, Olof Palmes Allé 43-45, DK-8200 Aarhus N, DenmarkDepartments of Endocrinology and Clinical EpidemiologyLeiden University Medical Center, Leiden, The Netherlands mso@clin.au.dk.
authors: Mette
authors: M
authors: Søgaard
authors: Departments of Clinical EpidemiologyEndocrinology and Internal Medicine (MEA)Aarhus University Hospital, Olof Palmes Allé 43-45, DK-8200 Aarhus N, DenmarkDepartments of Endocrinology and Clinical EpidemiologyLeiden University Medical Center, Leiden, The Netherlands.
authors: Dóra Körmendiné
authors: DK
authors: Farkas
authors: Departments of Clinical EpidemiologyEndocrinology and Internal Medicine (MEA)Aarhus University Hospital, Olof Palmes Allé 43-45, DK-8200 Aarhus N, DenmarkDepartments of Endocrinology and Clinical EpidemiologyLeiden University Medical Center, Leiden, The Netherlands.
authors: Vera
authors: V
authors: Ehrenstein
authors: Departments of Clinical EpidemiologyEndocrinology and Internal Medicine (MEA)Aarhus University Hospital, Olof Palmes Allé 43-45, DK-8200 Aarhus N, DenmarkDepartments of Endocrinology and Clinical EpidemiologyLeiden University Medical Center, Leiden, The Netherlands.
authors: Jens Otto Lunde
authors: JO
authors: Jørgensen
authors: Departments of Clinical EpidemiologyEndocrinology and Internal Medicine (MEA)Aarhus University Hospital, Olof Palmes Allé 43-45, DK-8200 Aarhus N, DenmarkDepartments of Endocrinology and Clinical EpidemiologyLeiden University Medical Center, Leiden, The Netherlands Departments of Clinical EpidemiologyEndocrinology and Internal Medicine (MEA)Aarhus University Hospital, Olof Palmes Allé 43-45, DK-8200 Aarhus N, DenmarkDepartments of Endocrinology and Clinical EpidemiologyLeiden University Medical Center, Leiden, The Netherlands.
authors: Olaf M
authors: OM
authors: Dekkers
authors: Departments of Clinical EpidemiologyEndocrinology and Internal Medicine (MEA)Aarhus University Hospital, Olof Palmes Allé 43-45, DK-8200 Aarhus N, DenmarkDepartments of Endocrinology and Clinical EpidemiologyLeiden University Medical Center, Leiden, The Netherlands.
authors: Henrik Toft
authors: HT
authors: Sørensen
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000209:etiology; D015331:Cohort Studies; D003718:Denmark / Q000453:epidemiology; D005260:Female; D006801:Humans; D006980:Hyperthyroidism / Q000150:complications / Q000453:epidemiology*; D007037:Hypothyroidism / Q000150:complications / Q000453:epidemiology*; D008875:Middle Aged; D012042:Registries; D012307:Risk Factors